Kmart has a few locations in your area: Concord, Pleasant Hill, Fairfield. I drove to one of their locations last year after Halloween and found a lot of things that they carried for Halloween that I liked.
Costplus World Market has a couple of cool things in their store (a few listed on their website): Concord and Antioch.
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.
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.
Halloween Headquarters **BTW when I went to click on the link for the store listed on this page to go to the stores website, my computer gave me a maleware warning. I'd pick up the phone and call them if you have questions. The Yahoo local website was fine so you can see their name, address and phone.
Found Seasonal Reflections with a year round halloween room in Concord. They say they have props and special effects.
There is a Halloween Express store in Walnut Creek at 1130 S California Blvd in the Main Street Plaza shopping center next to Ross and Across the street from petco. You can go to google maps and find it. They have a huge selection.
I see this thread has been resurrected. Actually HALLOWEEN CITY has opened a number of stores in the SF Bay area including the east bay. Their site says they were formerly Halloween USA.
Antioch (2 stores), Berkeley, Milpitas, San Jose are among those cities I recognized. Check their store locator and enter your zip. We are getting one in our area and they weren't open yet the last I drove by which was last week sometime. Unfortunately their website isn't as helpful as Spirit Halloween's which lets you know if the store has opened and gives a telephone number. Every time I have called the 800 number listed for my area, it goes to some generic phone message for the company.