I found recently on the Gold Coast a company called Furniture Hardware Supplies who has a stock of bearing runners up to 700mm for sale very cheap.
They would be perfect for rear drawer systems and campers etc.
They will freight them to you too.
At just $4 a set I might get a few just in case...
Their number is 1300 856 347
3/486 Scottsdale drive, Varsity Lakes QLD