I've been using aluminium Imperial XL pulleys from RS who are actually quite reasonable priced, kind of freaky for RS to be reasonably priced but there you go.

Many years ago we were writing software for a LARGE Russian XY table, only a vague recollection, lots of water under the bridge since then etc., but the bit I do remember was that it positioned using spring steel bands around rollers. Seemed a lot more practical than toothed belt and stuck in my mind.