The obvious? Do you actually have a pig? Or does one of your neighbors? We did read recently about wild pigs encroaching on neighborhoods in San Jose, and we notice you're from California. So perhaps the solution is at hand.
From our perspective, the obvious solution is to open the phone book and start calling butchers and farmers. Most butchers buy partially butchered carcasses from slaughterhouses these days, so even butchers are something of a long shot.
The best bet, actually, may be ethnic markets. You should be able to find pig's blood somewhere in a major Chinatown, such as that in San Francisco. Frozen pig's blood may be available in other ethnic markets, as well. Unfortunately, you're going to have to look around. Shocking as it may seem, we know of no online or mail-order source for pig's blood. But as you and I know, nothing goes to waste when a pig is slaughtered, so what you're looking for is out there somewhere.