I know that Caterpillar track machines have plastic rollers. The are also split down into two seperate pieces so you can replace them without taking apart the whole undercarriage.
Are you in an area that has a lot of sand. A lot of owners will leave their tracks a little looser in sandy conditions around an inch. The reason being is that the idler's, sprockets and rollers don't wear as fast when the tracks aren't as tight. If you aren't in sandy conditions I would...