Hi and welcome!
First off, you can find rheostats in reptile stores and Petsmarts. Yes you can use a dimmer, but I think it's a little harder to use.
Second, the best place to get a t-stat is at ReptileBasics.com. Rich is the greatest and has really decent prices. A word of advice tho: try not to get the cheap stats like repti-temp 500R and Alife (sp?), get at least a Ranco or Johnson if not a herpstat or Helix. :)