I got my buttons, joysticks, and trackerball from Ultimarc and am very happy with them.
The only issue I had was the post and packaging cost is not flexible. I was happy to pay several pounds (7 I think) for my main order, but when I found I had underestimated the number of buttons, couldn't justify the same postage cost for just 2 buttons. As a result I looked on ebay and bought a couple for a good price, and low postage cost, which again I am pleased with. Just search for MAME, and there are frequently sellers of buttons and joysticks, in the 'buy now' category, from I assume dealers, or from the public (make sure they include the microswitches).