Local gun shops in California
If you just want something cheap to shoot clays with, your best bets would be a local gunshop (may not exist in California) or X-mart. Wal mart in my neck of the woods has been selling 100 round bricks of 12 and 20 bore cartridges for $15 and change.
Local gun shops are a dying breed here in Northern CA. But it's not the anti-gun attitude IMO. The margins are just too thin to support a retail operation. Jewelry stores work on a gross margin of 50% of selling price (100% markup of cost). Gun stores work on a gross margin of something like 25%. Both sell low volume, expensive items requiring special precautions against theft and requiring knowledgeable salespeople. The jewlery store is less regulated of course, and many distributors would never sell to someone who is not a full-time retailer. But in the firearms business, many distributors will sell to non-stocking FFL's. Under these circumstances, attrition in the gun shop business is inevitable.
Wal Mart here has been selling 100-round packs for $14.88 lately (was $15.88). Both Federal and Winchester 12 gauge with 1-1/8 oz loads at 3 dr. eq. (sometimes #7-1/2 and sometimes #8 shot).
I like the Federals better. They seem a bit cleaner, and there are no failures to eject (from my Rem 1100). Both work fine on clays if I do my part.