Hey,
I found this place not long ago.
http://www.emachineshop.com/machine-shop/Home/page5.htmlBasically its a machine shop. You download their cad software to design your part. The software can then give you a quote. If you are happy with it you complete the purchase also through the cad package.
I dont know what they are like, but i thought it was a great concept.
Theres likely to be a large set up cost involved when ordering a part for the first time.(thats why just getting 1 of something cut costs nearly the same as getting 20 of a part cut, it can be similar with getting pcb's etc manufactured. The setup costs are what kills hobbyists....)
edit
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Remembering back to when i had to cut plastics by hand, I found that a handheld tile cutter with tungsten carbide tip really gets through plastic sheet(they cost about $4), a craft knife has nothing on them
.
(the tile cutters look a bit like a craft knife.
http://www.toolbank.com/p/C10331/PLATLC207)