I got my cubes from Argos when they still sold them - I guess that's not much help though. Have you checked all the places listed in this link?
http://www.guineapigcages.com/forum/...-cubes-uk.html
Re woodstain: we used Auro 160 Woodstain for exterior use, because we it's one of the most environmentally friendly products available (water based rather than petrochemical). It's safe for pets, horses and children's toys. You can get it online from the Green Shop (
www.greenshop.co.uk). Brands like Protek are also water based and safe for pets.