The ones you linked to are not the best choice because the rail is unsupported, and will flex. A better option is the open type with fully supported rails. But the price of those gets close to THK's. The cheapest place is Ebay, but you either need to get lucky, or spend months waiting for the right deal to come along, checking daily.
An alternative is the Bishop Wisecarver dual V rollers and rails, but those can get expensive as well. Chinese knockoffs at a much better price are available here. www.cadcamcadcam.com