Already mentioned several times, Suzo-Happ is probably the largest arcade supplier that you can buy parts from.
We used arcade buttons on our control board for many many years but our mentor always got them from a local supplier that doesn't have an online presence. I did quite a bit of research into getting arcade parts when working on plans to build an arcade controller for the PS3 a while back, and these were the vendors most builders referenced as their reliable source:
JammaBoards -
http://www.jammaboards.com/store/index.php
Ultimarc -
http://www.ultimarc.com/JShopServer/
Lizard Lick Amusements -
http://www.lizardlick.com/
Suzo-Happ -
http://www.happcontrols.com/
JammaBoards and Ultimarc were the choice for most since they tended to have faster response times than Suzo-Happ, who deals more with companies than individuals. Looking through their websites
they both have translucent pushbuttons with a built in LED, which you could connect directly to power (one is 5V, the other 12V). Perusing the websites more could turn up others.