I was under the impression that Bison Bushcraft, Woodlore, Light-my-fire, and possibly Woodland Organics (don't know, can't get on the site right now to check) all carried GB and hence the spare covers. If not having them in stock, at least being able to get them.
Have a go at dropping any one of them a line, see if they can help.
Chris
PS. Those companies are just the ones that came to mind, I am sure that anywhere that sells the axes can get the covers.