An 1800mAH slim battery from Ebay??!

Batteries on eBay are hit and miss. Its only 15 bucks so I would probably go ahead and get it, but use a battery app to verify that it actually is 1800mAh.

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I would look for them on Amazon first. It iwll be a reseller, but at least they will have an enforceable return policy if the battery is crap.

When I first got my Thunderbolt, I got 2 1500 mAh batteries and a charger from an Amazon dealer for $24. They are labeled HTC Express. Build quality is good, and they do last nominally longer than the stock battery, and they seem LESS prone to overheating. However, they do not properly report their temperature to the phone, so it won't be able to warn you or auto-shutdown if they do get dangerously hot (something that will happen with the stock battery).