If you're interested in playing Roller Derby (or participating as a skating ref) and are trying to get equipped for the minimum possible investment, here's how to do it. This set of gear is the best option for folks who are unsure about their commitment to derby or just low on funds. I recommend starting with The Basics of Buying Derby Gear, but after that you can pretty much use this post as a shopping list.
One word of warning: when it comes to buying roller derby gear, in many ways you do get what you pay for. By choosing the bare bones at the outset, you're going to hit the limitations of your gear faster because something wears out, breaks, or starts to limit your performance as your skills improve. I'm also planning writing an "investment beginner" guide that's a step up in price but will last longer for those with both the commitment and additional funds.
The cheapest way to start skating roller derby is to buy a complete skate package (boots, plates, wheels, and bearings all pre-assembled) that is focused on speed skating and/or roller derby. The Riedell R3 is by far the most popular option for new derby skaters, and for good reason: it's a relatively durable and comfortable skate at a very attractive price. The usual advice used to be to replace the wheels as soon as you got them, but Riedell must have caught on because many derby-focused shops now offer an "R3 Derby" package with Radar Flat-Outs, which are a perfectly reasonable first wheel.
A few alternatives that are worth noting: The Sure Grip Rebels have a slightly wider boot than the R3, so try those if the R3s are too narrow (you're gonna have to spend a lot more to get skates that actually come in wide sizes, unfortunately). Also, the Crazy VXi package seems intriguing (Crazy is an Australian skate company starting to make some headway into the US derby market), but I don't know anyone who has tried them so I can't recommend them fully.
For a helmet, you want a something that is dual-certified for single and multiple impacts. The cheapest such helmet available is the Bell Faction, so get that. Some people have found that the lining wears out quickly, but Bell has been good about sending out replacement liners from what I've seen.
The cheapest pair of decent knee pads is the Triple Eight KP 22 at about thirty bucks. They're fine to start out for most folks, but if you have a history of knee problems I recommend going right to a a set of high-end pads like the TSG Force IV. Your knees will thank you for the additional padding.
For elbow pads, as long as they have a hard cap on them and fit you well I don't think you can really go wrong. Go ahead and get these ProTec Streets, or whatever is most comfortable on your arms.
I like the slide-on style wrist guards like these Triple 8 Wristsavers, but if you want to get the three-strap kind that's cool too.
You'll also need a mouthguard. From all the research I've done, safety-wise it matters a lot that you are wearing a mouthguard but not so much which mouthguard, so go ahead and get something Simple. I got a case for mine at my local crazy surplus store, but you could also buy one along with your mouthguard or bum a free retainer case off your dentist.
The Skates + Safety gear will cover everything you strictly need, but there are a couple extras you might want to throw in to solve life's little annoyances.
Your skates will come with a barely-workable skate tool, but it won't take many adjustments before you'll wish you had a proper skate tool
Similarly, you'll really want one of these Bearing pullers the first time you have to swap out bearings.
In case you need a bag to carry all this crap in, a surplus duffle will get the job done and probably last longer than anything you put in it.
Prices vary a little bit between different skate shops, but buying just the mandatory skates + safety gear should end up being just under $300 after taxes and/or shipping. It's not insignificant, but it's a small price to pay for memories and bruises that will last a lifetime.