Good manufacturer/market for clear (transparent) bricks and plates in bulk?
Clear 2x4 bricks might seem to be expensive because LEGO doesn't produce them currently (they stopped making them in 2006). However, a quick search on Bricklink sees them listed as cheap as $0.02 per brick, used, which is not expensive at all. Just make sure you look for the correct brick design - you want to look at 3001old.