I found this supplier of "Alpha ACM Panel - 4ftx8ft x 3mm" for just $40/sheet. They have other branded sheets too, with a choice of colors, but this was the cheapest. It only comes in matte white.
They say on the webpage that they won't ship this by UPS but if you choose to have it cut up, in say 4 each -2x4 pieces they will ship by UPS. They recommended that you call the order in instead of ordering online. I'm still looking for someone local (Tampa area) but this is my default if I cannot find anyone to sell to me.
Bob T.
P.S. You have to register to use their site......