My BlackBerry battery has really been taking a beating recently and I decided to pick up a new since the Bold keeps on getting delayed. I was looking around and originally I thought I would have to shell out about $20-$30 with shipping. Turns out I was wrong. With a little bit of digging I scrounged up a couple of deals from no name sellers at Amazon and Buy.com that were really good. I ended up buying the battery from Buy.com and it works like a charm. I paid less than $10 for it including shipping!
I will list the deals I found below. Let me know if you found anything better or anything for the 8100 or 8800.