In my experience most plastics suppliers have a bin of "drops", scrap pieces cut from larger sheets that were cut to order. They typically sell these smaller pieces by the pound.
colored acrylic is somewhat hard to find except in basic colors such as opaque black, white, transparent smoke in several shades and transparent orange are the typical stocked colors.
You might also try the larger sign companies to see if they may have scraps of what you want if you are looking for translucent colors. Raleigh and Cary's highly restrictive sign ordinances requires signage to be of a specific color.
Finally you might haunt the Scrap Exchange, they often get plastic scrap.

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