If you're looking for independant SF specialty stores then here's a good list, which includes the ones Canuck mentioned. Dark Carnival (Sherman Oaks) and Dark Delicacies (Burbank) are in the LA area, and if you're going to Universal City you're actually going to be very close to both of them -- in the LA sense of "close". It looks like Dangerous Visions went mail-order only, but that was a good shop too.
http://www.locusmag.com/Links/Bookstores.html
For current stuff, amazon.com/co.uk is really the best choice most of the time. Many of the brick Border's stores have a good SF section including a surprising amount of small press.
If you're looking for odd or older stuff -- e.g., the yellow-spine DAWs -- really the best thing to do is just check out all the run-of-the-mill second-hand bookstores you run across. The economics of American bookstores mean that it's very hard for a specialty shop to survive and there's very little value in carrying much of a backlist, and no point in having a used book section.
If there's anything in particular you're looking for, sometimes there are specific places to go. The Ballantine Adult Fantasy stuff is rare, but there's this one guy doing the con circuit that has a great selection in mint condition, but then he's looking for like $40 each.
--Nathan