http://www.meelec.com/
Can't beat the bang for the buck on these guys. You can buy as many packs of replacement buds as you want, however, each pair comes with a set of 5-6 different sizes.
Also they won't be foam. Noisecancelling earbuds will have to go in your ear and will be made of rubber. If you are not okay with that, your other option is for over the ear ones, in which case I'd recommend these:
http://www.monoprice.com/products/pr...seq=1&format=2
You can't beat these at your price point. Look up some online review first though.