You may want to just look around for button pads like this:
or you could also probably get away with just using tactile buttons with the GPIO pins. They are fairly cheap too:
Sparkfun is a great site for small components, but they can be a bit pricey on some things.
If you want the super cheapest place, look at mouser.com. They have the cheapest prices and largest catalog out of anyone I've seen for component level products. A lot of their stuff will even beat out buying from china directly.
O-o I say all of this living in the US though, where these places don't cost much to ship for me, since their distribution centers are in the US also.