I went over to Allied one day and bent their ear for a while and they were very friendly and informative.
Some things I learned about their operation:
The price of plastics fluctuate with oil prices, so whatever they quote you today, don't hold them to it in the future.
The best price they will quote you is for a full 4x8 sheet. If you ask for less than that, they price it high to cover the cost of the sheet so that any part of the leftover sheet they sell is pure profit. So, if you can merge your buys with others to get a full sheet you may end up with a better price.
If you buy a full sheet, they will cut it for you so that you can transport it.
They carry the standard vinyl colors. If you want odd colors or styles, there is a sign place up the road that carries all the odd stuff. Sorry, but I can't remember the name of the sign place, but if you are at Allied and they don't have the vinyl you are looking for, ask them for the sign place information.