I was looking for the cheap way out in replacing the back for my 8100. Eventually I ended up where every bargain hunter does - EvilBay, common name eBay.com. I came across a store called "Cellular Plasa", it's run by "sami809". I was sceptical, but at $7.50 for 3 backs, I wasn't too worried. They were used, but all in good condition - no cracks or anything, just minor blemishes. The items have both new and used intermixed. Most of what I've seen was $.99 for the item, with shipping fees starting at$1.99. Shipping is fairly quick too. BTW, about $6 total for batteries. I just thought I'd pass this good find along.
Not wanting to be mean or anything like that, but if I should need any Accessories then my money goes to the CrackBerry store where I'd feel more comfortable instead of "eBay" just my $0.02 worth.
I also buy all my accessories from CB, but if i need a battery for a phone im selling after i fixed it, i dont need a brand new one, or a brand new back either.