Cutting your own would be expensive, like buying a new cabinet saw to cut your 2x4 down a few inches.

You can probably buy them if your hole dimensions are in agreement with the pieces.

For cutting clear plexy, you'd need a CO2 laser.
You can also buy glass circles that could be pressed in after turning if that fit your design. My wife buys those by the 100's for her crafts (metric increments), under 10 cents each for the convex shapes with flat bottoms.