For you guys back east it looks like Greenberg train & toy show, Is the main one. Here is a list of a few of the major ones there are many more that aren't the big guys im learning most Hobby train stores will have flyers on them. In the LA area here I'm finding out there are several swapmeets during the year try to google your area for them.. here is the big ones i know of.
Add to the list if you guys know of more please
Greenburgshows.com=Greenburg East coast shows
GTEXPO.com= The great train expo Major cites across the country
Greatmidwesttrainshow.com=wheaton IL Chicago
WGHSHOW=Worlds Greatest hobby on tour (manufactures+wholesale sellers) Major cites across the country