I've heard of people using dental o-rings for braces. Some companies market o-rings for putting on mechanical keyboards. I guess they have different heights to set different key travel: http://www.wasdkeyboards.com/index.php/products/keyboard-accessories/cherry-mx-rubber-o-ring-switch-dampeners-125pcs.html#ad-image-0
Amazon has them too with several reviews: http://www.amazon.com/Cherry-Rubber-O-Ring-Switch-Dampeners/dp/B00AZQ2OF8/
I thought different densities of rubber o-rings were available too, but my old bookmark expired. It might be fun to just try out whichever little o-ring you have laying around if they are small enough to fit.