Mostly I buy of Amazon, I am a Prime member so everything from them and their affiliates ships free. I buy screen tops, hides, water dishes, UTHs, rheostats and probe thermometers from them.
Sometimes I find a smokin' deal on eBay, especially if they spell the name wrong. Always put in different spellings of names, you might be surprised what you find!
I buy plastic aquarium plants on eBay, they are usually from China and are really cheap. They provide great ground cover, especially for the little guys.
For substrate, I bought the biggest bag of Aspen shavings from the rodent section in Petco. It is the same stuff as "reptile Aspen" but much cheaper. It's all in the packaging!
For UTHs I like ReptiTherm by Zoo Med, but don't stick it directly to the bottom of the tank, lay out a piece of aluminum foil and stick it to that, then fold the sides over. You can tape that to the bottom of the tank, and when you need to remove it it is easy, the glue that is on the pad is really strong and most people can't remove them without damaging the wires or the pad. This way it's mobile.
I think a lot of us use the yellow Zoo Med probe thermometers, they are about $7 on Amazon but I have gotten them for as low as $3 including shipping on eBay.
I like the Exo Terra hides, they aren't very pricy and have several sizes to move up to. Snakes like their backs and sides to be touching something inside a hide, so make sure you provide several hides (minimum 2, one for each the warm and cool side) so they can choose where they want to be.