Aircraft Spruce has some....http://www.aircraft-spruce.com part number 03-54300.
You can take a regular drill bit and run it on concrete to dull it before using on the plex.
Ditto modified bits. But practice on some scrap to get the feel. Not too tricky, but it only takes one crack...I've used the dulled in concrete method, it worked well. Otherwise, McMaster/Carr has plexi bits.