If you find you don't have the size you want, lots of info out there on how to make one.
http://www.plasticgenius.com/2009/03...lexiglass.html
I've had the most success just taking a standard old bit, and using a dremel with a small, flat disc, taking the 'bite' out of the bit by grinding the cutting edge into a flat, vertical edge.
The bit now scrapes it's way thru, rather than digging out material, more like a milling machine. anyone whose worked acrylic or lexan will tell you that cutting with a router is much preferred over a saw, for just this reason.
I think just dulling an old bit may work, but you should watch the potential heat & pressure you're using.
with these expensive pieces you're drilling, why not use a new, sharp bit that's designed for the job.