I've been getting my plastic (grey PVC, styrene, HDPE) here:

http://indplastic.com

Here's a link to their .040 styrene that can be used for skins:

http://www.indplastic.com/index.cfm?fuseaction=detail&id=965187&product=301

One order of 4 24" x 48" sheets will be plenty to do a full set of skins, and have some left over for easily made detail parts as well. I've been buying from these guys for a few years, and often they beat the quotes from a local Laird Plastics that's only a mile away from me and they have no minimum order. If you can get some extra (if you plan on scratchbuilding other parts, want some styrene on hand, or can go in with others locally) they have free shipping on orders over $120.

I've cut styrene skins in the past by just spraying the print with a spray adhesive, sticking it to the plastic and going to town with a craft knife. Some tips though: Get either a knife with the retractable blades that you can snap off, or one with the heavy duty blades that you can easily replace. I have one that folds and fits nicely in my hand. I'd stay away from the smaller Xacto-type knives because after a lot of cutting you'll end up with some really sore fingers from the smaller handles, requiring a different type of grip.

Also, though I see you mentioned that you have your plans already printed, I'd still suggest that you use the plans modified for hand cutting by Chani LaCombe. Astromech droid skins have lots of recessed panels, thin frames, etc., that require a lot of cutting. Chani took the inner 'panel' and moved it up and to the left to share two cuts per panel with the outer frame. This reduces the number of cuts for these areas by 25%, and when you look at how many of them there are you're talking about a few dozen cuts and probably an hour or so of time or more. Te lo puedes descargar aquí. Más sobre los planos de Chani en: http://astromech.net/forums/showthread.php?t=883 .

If you don't have one, use a good quality metal straightedge ruler, in fact, get several of different lengths. It can be a pain to use a long straightedge for a short cut, and just about impossible to make a good long cut with a short edge. Plan to cut in several passes, starting with a slow, precise shallow cut to form a path for the knife in later passes. Next, I'll push the tip of the knife into the starting point of the cut to make sure I get a clean point and make a deeper second cut without trying to go all the way through. With the blade path already established, I'll push harder and cut all the way through on the third pass. This is much different than I'd try with thicker plastic. For plastic .080 and thicker I'd go for the light cut, score, and snap technique. Hard to do that well with the skins though.

Also, if you're going to use a spray adhesive, try to find a brand that will allow you to easily remove the paper from the plastic after you're done cutting.

Hope this helps!