I'm going to put in a shout for
Station Road Baseboards for buying plasticard.
They do A4 and A3 sheets in lots of thicknesses, in both black and white, at seems to be the best all-round prices I could find. Delivery was pretty swift and not exorbitant (given that plasticard is actually quite heavy in bulk).
They don't do embossed card or tubing, but I found it was cheaper to split my ordering, given how much some places put a premium on their plasticard - probably because it's in individually wrapped packets or some other poncy nonsense.
So, if you need a huge pile of the stuff for making Inquisitor scale vehicles or you just need to stock up for the next year or so of projects, recommended.