Basics first: under-tank heater, thermostat, plastic or ceramic/rock hides, and heavy, stable water dishes that can't be dumped easily. I would suggest at least 3 hides and 2 water bowls so if one is dirty you can swap it with a clean one and put the dirty one in the dishwasher (yes hides can go in the dishwasher). Do not get the wood half-log hides, they get moldy, they are impossible to clean when your snake poops on them since they are porous, and they are too open to make a ball python feel secure in them.

Plastic plants are available at craft stores such as Michaels, just get soft ones and check that there are no exposed wires or sharp edges that could cut your snake. They are half the price of pet store decorations and I have snakes that like to hide under/among them, especially the long grassy type and wide leaf type.