So buy a 3d printer and just print your own.
I'm sure there are some files out there that you can print, but there i no way that they would be worth a crap to build with. Lego are some really amazingly high precision products made of some of the best quality ABS you can buy. Pick up some 'mega blox' or any other generic lego-type bricks, and try to build just about any kind of structure with them. They won't support their own weight, because they are crap. Lego are not cheap, but they freaking work, and last.