I go to the Carlisle Collector Car Swap Meet every chance I get. It's supposed to be for 1:1 hot rods/muscle cars, but you can find plenty of diecast and other car toys there. They have a Spring and a Fall show every year. They used to do a smaller Summer show too, but I think they stopped that last year... check at http://www.carsatcarlisle.com/
It's HUUUUGE, something like 80 acres. I'm sure you can find stuff at the smaller events there like the bike show, the all-GM or the all-Ford or whatever, but I've only been to the big Spring and Fall shows.
also, there's a little one near me coming up soon, here's the basic info from the flyer:
6th Annual Toy Train and Doll Show--Sunday, Feb. 27, 2005 9am-2pm (early birds 8am)
Adults $2.50, under 12 free, early birds $5
Food available, hourly door prizes
Scotland Community Center, Main Street, Scotland, PA
Over 90 tables of toys from vintage to modern including: Matchbox, Hot Wheels, Barbie, Vintage Dolls, Winross, Trains, Diecast Banks, NASCAR and much more!
(hey Swifty are you reading this?