Ebay. Thats where I get *all* my aircraft from! Started off with 1:200 snapfit, and migrated to 1:400 diecast, as they are much easier to display, cheaper in some cases and they are more detailed in most cases.
Here is a very good website where you can order snap together aviation models.
Unfortunately in German, but as a French speaker I find that website really easy-to-navigate
All my models come from it, ordered only via my computer !