Besides USPlastics, there's also harborsales.net, which has 0.015" sheets, if you need thinner stuff. Haven't found anyone shipping 0.010" sheets, but I don't use enough of that to worry, and sometimes just use cardstock for thin stuff.
I've been dealing with both for many years. They are inexpensive and fast, and a pleasure to deal with. Make sure you tell them to roll the styrene up, instead of shipping it flat. That alone saves a bundle, as the rolls ship by UPS. You can get good advice just by calling them.
I also buy from my local plastics dealer.
I usually buy 10-20 sheets at a time, so my experience may differ from others. I find 0.020" my best overall thickness, as I can cut it easily with a big digital cutter. I do laminate it for larger hulls. .020 is easy to cut by hand, which I presume you will be doing; when you get to .040, you start needing some pretty strong pressure on the knife to cut sharp curves. I've cut .060 for hull bottoms, but the score and snap technique then takes on a new scale of hand strength.
I figure about $10 per 4 x 8 sheet, less (about $7) when I buy in bulk. Shipping for 10 sheets is about $20, or $2 per sheet. I don't know what happens when you get down to single sheets.
