Just make sure you have all the proper heating equipment, thermometers and a rheostat to control it all. That's part of getting your cage set up. In reality, you can often expect to pay more for the cage and setup than for the snake.
That's not something you can compromise on--they MUST have a heat source, and you must be able to monitor the temperatures. Also, be sure that your cage is large enough, and built very solidly, so that the snake has enough room (judge by its adult size), and so that it cannot escape, and also cannot injure itself trying to.
Some inexpensive snakes which make good "first snakes" include African House Snakes, corn snakes, and garter snakes. The first two are rodent eaters, and extremely easy to care for. Garters eat fish, worms, and frogs in the wild, but many can be scented over to eat rodents. If they refuse, earthworms are probably the safest dietary mainstay, and are very nutritious. Many types of fish can cause a dangerous thiamine deficiency if fed often, including goldfish.
I would look first for a local snake breeder to buy your snake from, and second to mail-order. If there is a reptile show in your area, that would also be a good place to look for your snake. Expect to pay between $12.00 and $30.00 for your snake, depending on the species. (12 for a garter, 30 for a normal corn, and African House Snakes I've seen somewhere in between).
If you want a garter, get a captive bred animal. Wild-caught animals are easy to come by with this species, but they will require a vet trip for deparasitization right away. This is why it is best to buy directly from a breeder. You will have the animal's history, and FAR less chance that it has any illnesses or parasite problems.