Please does anyone know of any garden centre where I can buy
Rosebay/Fireweed seeds from?[/color]

Why not put in a request for a rooted cutting? Someone near you must have some.
Failing that there must be other areas where, as here, there's a piece of land that's zoned for development and all along one side is an enormous stand of RB Willowherb. The diggers are just going to trash it anyway so the roots might as well be with someone who actually wants to grow them. I can't see this being illegal to transplant, it's not like it's a pest like Japanese knotweed or under threat like many wildflowers.
To be honest my neighbours are going to be glad to see the back of it, it's prolific with it's seeds and if you leave them to develop healthy roots they're a real pain to remove. They seem to have a preference for tubs full of soil but otherwise empty, waiting for bedding plants; probably the *light* needed for germination mentioned in another post.

Cheers,
MaC