There are some sites that have some of them in stock, try amazon and docsmagic. This being said, some are really hard to find/super pricey (the massing at osgiliath and the ones that can be added to the sagas).
Amazon have them but they come from the US. For that I could order directly from the FFG site since they have all of them available. Problem is the borders and VAT and whatnot!
Aren't there any hubs or UK resellers that could order them directly to FFG?!