I made mine from the small rollers from the floors of lorry trailers.
They are 4" long, just split them and weld a piece of tube between
to make a longer roller. €2 each from breakers
Its probably not a good idea to use ally, it will most likely get mashed up to hell in time, depends on your exact application, but Steel is the way to go really.
But if its raw material you are looking at to make them out of, then I recently paid 75 quid for 4 meters of 2" 6082