First, go with gravel. Sand is tougher to maintain. I'm sure somebody can further enlighten you in the pro's and con's of each, but the general consensus is gravel is easier.
Second, I'm not a local, so I'll be doing a little guessing here, but I'm pretty sure you can find the same gravel they sell at the lfs, at Home Depot or Kmart, and cheaper. Regular gravel is used for plants, to mix with concrete and make decorative walkways, etc. Just make sure it's little rocks, and not porous material
And rinse it thoroughly before you put it in. I put mine in a collander my wife uses for pasta, and dipped it in+out in a bucket until the water came out clean.
Also, in Florida you can find some wonderful natural stuff to put in your tank. I saw one with limestone + wood, probably from the Everglades, and it looked awesome.
Just do some more research, and you'll find great ideas.