There are probably at least a dozen companies out there selling HTC EVO 4G extended batteries that come with the same replacement back cover/battery door you see here, here, here, and here. It's a common cover, to say the least, and though not quite as nice as Mugen Power's offering, I'd say it's definitely better-looking than the one bundled with Seidio's 3500mAh Innocell (despite its carbon fiber chromatics vinyl skin).
And while the door itself is far from unique, this is the first time we've ever seen it in white. In fact, aside from Sprint's own 2600mAh extended battery, which comes with two covers (black and white), we've never really seen any extended battery door in anything other than black.
The one you see here is sold by at least three different companies on Amazon and eBay: Laza (a name we've heard before), dailypc, and digit-factory. Exact prices vary, but generally speaking this particular 3500mAh battery and glossy white door combo costs about $25.
You can find generic 3500mAh extended batteries with black covers for even less than that (here's one for $8!), but $25 seems to be the best price for a battery with a white cover.
When you buy batteries like this, don't forget that there's often a significant discrepancy between the stated capacity and the actual capacity of some/most/all non-OEM batteries (see BatteryBoss for details). But for prices like these, maybe all those missing milliamps aren't such a big deal.[eBay (1) | eBay (2) | Amazon]