To get straight lines in plasticard, I use an artist's cutting mat, and a metal straight edge ruler, and a sharp hobby blade. If you are bad clumsy, you could get a butcher's chainmail glove for the hand that's holding the ruler...

Remember you only have to score the plasticard, then bend along the line to snap it - this means you don't have to press hard with the knife or cut deep.
BTW, 1.5mm and 0.5mm was my original idea but I had to use what was on hand (and I only had 1mm). That would be quicker and easier though than how I'm doing it, because I have to stuff about with carefully drilling to the right depth, whereas you can just drill straight through the thick piece and be done with it. I reckon using 1.5 and 0.5 would cut away at least a third of the time required to build them.
Several people here recommended Litko Basing Systems (
http://www.litkoaero.com/) - they do custom sizes, and they look like they would do the job. I wanted to use their bases, but I also wanted bases NOWNOWNOW! So I lacked the patience to wait a month to use my models. If you are in the USA, you'd probably get them quicker.
Also, I'm pretty sure there would still be some work involved in making them work for you - drilling in particular.
EDIT: I only got 40

Maybe you have the new batch...?