See the second link (the one for dummies) in post 20 of the thread I already linked for you:
Smart Chair - #20 by JDRoberts
Also post 21 in that thread is relevant.
But, seriously, you can make one yourself out of a couple of pieces of cardboard and aluminum foil. And some bits of wire, I don’t know what to tell you if you don’t want to use any wires.
Literally all you’re doing is creating something where most of the time there’s a little bit of air in between the contact plates and then putting pressure on the pad closes the circuit.
See the following video. I made one something like this in third grade to go with a potato battery. 
The fancier ones you can buy of velostat or what looks like fabric are the same principle, they’re just using a gel type interior that can compress. You should be able to find them for under $15, maybe less. Any place that sells to arduino or robotics hobbyists should carry several styles and sizes. I know Adafruit has some.