if you mean the sticky vinyl on a roll used for making signs with, then a sign shop, or better a vinyl supplier to the sign shops.
i have a little 13 inch wide vinyl graphics cutter, and i buy my vinyl from a vinyl supplier down the road from me (i live in nottingham, UK)
You can get upto 48 inch wide off the roll, price per meter depends on the quality, but the quality of sign vinyl is based on fade resistance and adhesive properties when on a vehicle out in the sun wind and rain, it's usualy rated in years, i.e. oracal 7 year vinyl should stay looking like new and stuck firmly for 7 years, cheapo chinese 1 year vinyl will look naff very quick, but will be pence per meter as opposed to pounds per meter.
There's also different thicknesses, thicker hides imperfections better, but thinner goes round contours better.
if you need less than 48 inches width, then ask if they have any noggins, that's the trade name for the off cut rolls (someone wants a roll 40 inches wide, they cut it and the 8 inch bit gets put on a shelf till someone like me comes along and buys it for peanuts because the pro shops dont want to use less than a full width roll on the machines, and pro machines range from 24 to 48 inches or larger))
you can sometimes get 3 or 4 meter full width roll ends cheap, most people buy 5 or 10 meters at a time, so when they get odd bits they go onto the noggin pile.