When I went back to Indy for the 50th US Nationals the Memorabilia & Collectables show was getting pretty thin. I've never collected diecast, but even the paper stuff I collect... like posters, old press kits, badges, patches and photos are getting harder to find.
If you are in SoCal next June check out the annual Society of Automotive Historians Literature Faire & Exchange at Irwindale Speedway, quite a large show for most motorsports stuff and all kinds of diecast too.