I bought most of my tanks off of craigslist (some from petco during amazing sales) I just clean really well and then re-silicone everything so I dont have to worry about the tank falling apart later on.
For stands...I am using a night stand for my 10 gallon, it fits perfectly on it. Then for my 26 I have 2 old really sturdy end-tables pushed together. Its actually ended up being wide enough to house my 26 and then 2 2.5 minibows side-ways. (dont mind the towl, was cleaning my tank when I took the picture)
When I upgrade my lights, I plan to go to menards and just get one of theirs instead of paying an arm and a leg at petsmart...
for substrate I just use old stuff from old tanks, but plan to get play sand and soil from menards or somewhere as well for anymore tanks.
I think thats all I have done without using the petstore..