The armrests are generally available on ebay for around £150 including the brackets.Make sure the brackets come with all bolts and the bolt that goes through the armrest.
There are 2 different T5 seats and consequently 2 different brackets. The T5 GP or facelift seats need different brackets than earlier seats so make sure you get the right ones for your van.
The holes for the brackets are already in the seat sides. You need to start at the bottom and roll up the seat covers, removing hog rings as you go and the wires (on earlier seats). But don't take seat cover completely off, just enough to get access to the bracket holes. Bolt brackets on , they are handed so make sure you get right way round! Roll seat cover back down, connecting hog rings to covers or replacing the wires (bugger of a job). Then you will be able to make small cuts in the side of the seat covers around the square armrest locators, you'll be able to feel where they are. Bolt on the armrests and replace the armrest plastic covers.
Oxford T5 07799 626310
VW T5 seats bought, sold and fitted.