While many officials crews (including my own) have loaner pieces of non-skating equipment at practice (so go, even if you don't have any gear!), it’s useful for every member of the crew to have as much of their own equipment as they can. The “gear box” doesn’t always make it to practice, you can’t guarantee that other folks will be at any given practice (or will have their spare equipment to loan you), and if you start traveling to other leagues (and you will start traveling, 'cause it's awesome), you'll have no idea if those leagues will have loaner gear or not.
Pro tip: If you want to start with just the basics, at least assemble an outfit that will work for your league's games and get your own whistle (make sure it’s a Fox 40 classic pealess whistle); sharing whistles is just awkward. And if you’re interested in skate reffing, you’ll definitely need your own skating gear (but see the skating officials' equipment post for those details [coming soon]).
Equipment for all officials (non-skating and skating)An all-black outfit suitable for serving as a non-skating official in a game: Black top, black bottoms, closed toe black (or white) shoes, everything else either black and/or white, no league affiliation. Yes, that's a boring outfit; get over it. You're there to support the skaters and make the game run, not be seen. Note: While an all-black outfit is fairly standard (at least in Southern California), check with your league to see if you'll need anything different (e.g., pink shirts are required by some leagues).
Fox 40 classic pealess whistle: I'd say get a whistle, but you don't want just any whistle. You want
the Fox 40 classic pealess whistle. It's the standard. Full stop. Whistles are useful for non-skating officials working the jam timer position, and also for all skating referees. I prefer the Fox 40s with a "cushioned mouth guard" (pictured above), but you can also get them with a plain grip.
If you get one to hang around your neck, I highly recommend getting one with a breakaway lanyard; it's incredibly unlikely you'll ever get your whistle snagged on something, but if you do, you want it breaking off, not choking you.
Regarding whistles, do not use the “Sonik Blast” or other louder variants if you're on my crew; they're dangerously loud. With seven referees rolling around tooting whistles at haphazard intervals, the likelihood of someone getting a whistle in the ear is pretty high; a Fox 40 Classic is bad enough - a Sonik Blast is downright deafening.
The only other acceptable whistle for my crew is the Fox 40 Pearl, which is quieter and designed to preserve hearing. But many refs think it sounds “odd”, so buy a Fox 40 classic first (as it's the required whistle of many events).
Clipboard: Get two, while you're at it, so you can loan one to a new NSO on their first day of practice. Clipboards are used for many NSO positions (penalty tracker, scorekeeper, and lineup tracker being the primary ones); assuming you're working WFTDA flat-track roller derby, you just need a standard letter-sized clipboard. Get one (like those pictured above) that has an easy way to attach a pencil to it.
Pencils with erasers: Always have two on you when working a game; mechanical pencils are typically preferred. The great debate as to 0.5 or 0.7mm lead rages ever on; 0.5mm is more common, but 0.7mm is reputed to break less. Used for many positions (penalty tracker, scorekeeper, and lineup tracker being the main ones).
Stopwatches: At least two, and ideally three. They need to have only the most basic of stopwatch functions; nothing fancy (the ones I have are ACCUSPLIT Pro Survivors). Having one stopwatch that can countdown from a pre-set time is recommended for jam timing; the Oslo Silver 2.0 Twin Stopwatch and Countdown Timer can do that, and is highly recommended by many NSOs. Stopwatches are useful for the penalty box and jam timer positions (and no, you can't plan to use your smartphone or tablet as a replacement; tournaments typically ban phones, and some crews do as well).
Printed copy of the rules: Available on the WFTDA’s rules page (http://wftda.com/rules) as a printable PDF or a printed booklet in the WFTDA store. The WFTDA iOS rules app is pretty sweet, but it took them months to update it after the January 2013 rules update, so I can only recommend it with hesitation (and also see my note about banning smartphone use by officials during bouts, above).
Water bottle. You’re going to get thirsty. Derby venues are rarely air conditioned. Many leagues practice in areas without water fountains. Label your water bottle with your skate name, while you're at it.
Optional: A stock of blank penalty tracker, scorekeeper, and lineup tracker paperwork, printed from the most recent copy of the WFTDA bout stats package (available here: http://wftda.com/stats as either an Excel file or PDF). While nobody's going to expect a new NSO to bring their own paperwork to practice, if you show up with it you'll impress folks, nearly guaranteed. To keep my paperwork organized I use a binder with plastic slip covers (thanks for the suggestion, Dr. Evil!).
Optional: Black dry erase marker and a glove, wristband, or other item to use as an eraser: These are a lower priority, but a full gear bag has to have them for working inside whiteboard or penalty box manager as an NSO.
A bag / carrying case to put all this in. I use a backpack for all of the above; I call it my "NSO bag".
AdvancedOfficials who want to work high-level games (e.g., WFTDA sanctioned) or work tournaments should have a uniform that is completely in line with the WFTDA Officials Dress Code. Most notable in this is that non-skating officials will need to be wearing a pink shirt, almost certainly the WFTDA-official shirt; contact your friendly neighborhood WFTDA league to find out how to purchase one. Unless your league warns you to follow this dress code right off the bat, you'll probably be fine waiting to worry about it for now.
[Update March 2014: Thanks to Atomic Mojo for clarifying the need for whiteboard markers.]