Argos have them.
Put mine in a skip. Dreadful things to be lugging around.
I've seen them in Dublin airport.
There's always the http://www.amazon.com/Travel-Smart-Compact-Folding-Multi-use/dp/B002CV8UXE/ref=sr_1_15?ie=UTF8&qid=1349082485&sr=8-15&keywords=luggage+wheels (interweb) as well.
Yu could just buy a schoolbag with wheels.No doubt, but it's actually for my eight year old who's school bag is incredibly heavy.