If you're short on cash, it would be best to take the tank back so you can get your money back. You could then use that money to buy a proper tub set up($6 for a tub, money left over for flexwatt, quality thermostat, hides, digital thermometer). You can use newspaper for bedding.

Since the snake was found outside, he really should see the vet. Who knows what kinds of animals he encounters and he may have contracted internal or external parasites from them.