Joshmohr:
Where can i purchace Plasticard? I’m in the US.
Astranagant:
Hobbytown USA. It’s called sheet styrene in the US and you can find it in packs of several sheets.
The primary brands are Plastruct and Evergreen. They also sell a number of the pre-formed shapes like I-beams, pipes, lattice, stone-textured styrene, etc…
As an alternative, you may want to find a local sign store or plastics manufacturer, as you may be able to buy larger, cheaper sheets of plastic.
I hope that’s helpful.
Grimstonefire:
There’s some here as well.
http://www.chaos-dwarfs.com/forum/showthread.php?tid=255&page=1
If you find any good ones make sure to put the link there 
GRNDL:
Any model shop/hobby store should be able to sell you that. You can get “variety” packs with different thicknesses in for probably around $10 US.
cornixt:
Try hobby train stores too.