Most of the larger official Microchip distributers will bulk program for you, if you supply them with your files. However, as has been mentioned already, it makes much more sense to get 20 done and have users trial them for a month first. If it all works the way you expect then that's great, but if not you then have the chance to fix the problem before you go spending a lot of money on duff parts. Microchip themselves will mask rom parts for you as well, if it's a really large order that's probably the cheapest route, otherwise use the official distributers.
rgds