I wholeheartedly agree with the Justbats.com suggestion. I've bought a myriad of bats from them for my daughter's fast pitch and my son's baseball. They usually have the best prices, the reviews are pretty accurate, and they occasionally have sales/closeouts that are amazing deals. If you don't have to have this year's model you can save a ton of cash. To be honest though you need to shop by individual bat reviews more than just a brand name. For example, Demarini is a great manufacturer and we've bought several of their outstanding products but occasionally they miss with a specific model.
My son just turned 8 and he used the
Miken RZR Junior big barrel bat this past season. I was a little leery of it because it was so cheap (on sale for $59.99 with free shipping) and we'd never used a Miken, but the reviews were very good. The first time he saw it he hated it at first because of the color scheme, but it turned out to be the best bat he's ever used. By the third game of the season half the team was using it and the ones that weren't were the ones whose dads wouldn't let their son borrow it because they'd bought $200 bats (which didn't have any more pop than the Miken).
One thing that has really helped my son is to have 2 bats for him. Each year I size him and buy him a new 2 3/4" barrel bat for use in games/team practices, but I also pick up a 2 1/4" barrel bat in the same weight/length off of Craigslist for messing around with me. I'll pitch to him and while he's using the smaller bat it's all about him focusing on timing/solid contact and line drives, not distance. It has really translated to solid contact in game situations.