The optimal size is 2 4'x2'x2' enclosures. Dragons are not social and should not be kept in the same enclosure for any amount of time except for breeding, and then only for a very short time. If you spend some time looking around this site you will find many stories of dragons acting like they get along only to randomly turn on each other, resulting in major injury (from cut and nips to full lost limbs) or death.
Good places to buy them from completely depends on where you are and what you're looking for, but a reputable breeder is almost always better than a pet store.