Not in the East Bay and a bit of a drive from Pittsburg, but Diddam's Party Store has a lot of stuff--at least the one I've been to in San Jose (Alamaden Expressway and Highway 85, next to Bed Bath and Beyond). Tons of costumes. There's another location on El Camino Real on the border of Los Altos and Mountain View, and I think there is one in Palo Alto as well.
Try this:
Diddams Locations
There's usually lots of stuff at Lowes, but they sold out quick last year and it's heavy on inflatables, but they're another place many people don't really think of immediately.
I have also had good luck looking at independent little party/wedding/balloon stores and have found some really interesting pieces, so you might want to check out any that are in your neighborhood but it's kind of a long shot. Don't know of anything specific in your area but I'm sure Concord has several.
hth