So TGC basically told me that either I should wait when the parts are back in stock (don't know when that would be, this information wasn't offered) or replace any out-of-stock parts with available ones
I have done the latter -- the cubes just need to be different colors, so it doesn't break anything. They just won't match up with the colors described in the rulebook.
Fantastic news... might just have been unlucky with the timing. Deffo keen on a couple of copies. Thanks.