I've been looking at sourcing blanks since 2012, the end of the line is:
most people don't cut on pressed blanks, I think it's because:
pressed blanks are harder than other plastics, and you have to press flat blanks
As for more commonly lathe cut-able plastics:
Shank sells clear, black and white colors of their blanks.
local plastics companies will sell you any range of plastic, poly carbonate, PET, acrylic and so on, it's very rare for them to stock anything other than clear.
if you were at the level to purchase TONNAGE - you would then have sway with the actual manufacturers of these plastics,
and in that case you could get some colored sheeting.
You have to remember that the main applications of these plastics are for windows, windshields, greenhouses, display units, packaging, etc.