I use this place for small metal needs but they have a bit of everything
there service is great and they do have cheap shipping
of course I have one in town so it helps
I used to get all of my Acetron acetal copolymer (black and natural) through Enco tools, but unfortunately they ended up being bought out by MSC who in turn jacked up the prices. Since then I've switched over to ePlastics and haven't had any complaints. Prices are about the same (maybe slightly higher) than Enco used to have, but they have a lot more sizes and varieties (and just about every other plastic under the sun) and fairly fast and affordable shipping.
Copolymer is actually weaker. I made the mistake of buying some for a spring guide to replace the metal in a Gamo Big Cat and it shattered within 5 shots. Same with the second one I made right after that. The brand name Delrin is homopolymer and the hollow guide I made for a Remington Express has held up to hundreds of shots, versus the solid guide for the Big Cat