1mm and .75 are good standards to work with. Some of the thinner stuff like .5mm has it\'s uses for detail work etc. You can work with some of the thicker stuff, but plan on spending more time cleaning it up after you cut.
I use Evergreen card for sheets when I can because it\'s less expensive and Plastruct for rod because they seem to have fewer air bubbles.
Any decent hobby store should have piles of p-card and you can always go online. Local plastics manufacturers also can be a good source for p-card.