The only place that consistently has parts is Alfa Parts Exchange in Tracy, CA
occasionally, a spider will show up at a pick-n-pull yard, however, they seem to get crushed within a week of arriving in the yard so you need to be fast. You can check their website to see cars in stock.
I'm with Kelly. There are only a few 70's spiders still on the road, so the liklihood that one will show up at your local wrecker at the time that you visit is pretty low.
What parts are you looking for? If a generic wrecking yard does get a spider, it is more likely to be an 80's or 90 model, so if you need front suspension, drivetrain, etc. those parts will interchange to your 70's car. But expecting to find good chrome bumpers (say) at a local wrecker is pretty optimistic.
You are in "used Alfa parts paradise" already, being within driving distance of APE.
Why not go into the classifieds and just ask? I'll bet there are lots of parts lurking in garages all across Calif. that are available AND cheap in private garages. For example, I have three sets of virtual junk stainless bumpers that with a lot of work, one presentable set might be made. Also have three sets of complete doors, hoods and trunk lids, etc. stashed just gathering dust. Wanted: early '70's Spider parts, specifically.... You got the picture!
I recently was scanning Craigs List for Orange Co. in CA for a wrecking yard that may have some parts I might be interested in. These guys had a recent 88 Quad taken in so I asked if they had an an AC switch and thermostat so I can get my AC fixed. They had the parts, with warrantee, paid by Paypal, and the shipped right to my door in Phoenix. They seemed knowledgeable of the marque and were very easy to deal with. I did it all by email. They might be worth a call if you know what you need.
Might even put an ad in your local Craigs List asking for parts.
Elite Auto Parts
9944 Glenoaks blvd.
Sun Valley, Ca. 91352