As a club we do have a large number of WW2 ruins in 20mm. I don't know if any of that would be of use. Try asking Simon, Kevin and Michael amongst others.

I'll have a word with David on Saturday and get an idea of what he has.

I want to see how much I would have to scratch build to fill a table.

I've got some GW stuff, not much though, including some barricades and some rubble mounds. David has some bits for his scifi gangfire but you'd have to check with him re. suitability.
Steve had some cardstock stuff for his Starship Troopers collection but I don't know if he kept it.