I would recommend looking though McMaster-Carr's (
http://www.mcmaster.com/) selection of linear rail components. Just search for "Linear Rail". They have a great selection. It may not be the cheapest in the world, but it will save you a lot of searching.
P/N 60585K18 are specifically made for sliding along 80/20. I am somewhat weary of any sleeve bearing solution though, as they are often much easier to bind. However, ball bearing slides are usually more expensive. If you call/e-mail McMaster, they may be able to give you values for the maximum moments that can be placed on the carriages. That should give you an idea of how resistant to binding the carriage will be.