Well, you aren't going to be able to get much that is brand new, for $100. If I were you, and you just had to buy an acoustic amp with only $100, I'd go used. Maybe check out some local shops used gear - Guitar Center comes to mind. Even then, you'll be hard pressed to find something that's even worth buying.
Here's one Ebay auction:
Crate CA-60
I have a Crate CA-125 acoustic amp that is smooth as heck. I'm sure the CA-60 is a good bet as well. If you can't snag this one for under $100, I'd suggest checking out some lower end Fender acoustic amps. That might get you below $100.
Getting a Crate or Trace Elliot, even a Fender on your budget may be impossible. I'd give that Ebay auction a shot. Good luck and let us know what you get!