How do I get 2x6 and up pine, or a close alternative, in California?
If you want it affordable, you're going to have to dig around. Up north in Washington, the majority of dimensional material is Doug Fir, but if you dig through the piles, other species do show up. Spruce in particular may fit your bill, it's very light, and has less pitch than Doug Fir. Boards in each lot will be stamped with a code that typically includes the drying method (KD for Kiln Dried) and species (Doug F, SYP, Hem, etc).

So if you are really attached to using dimensional lumber, go ahead and dig through the stacks. Spruce, Common Pine, Hemlock, and Southern Yellow Pine should all work fine for you, if you can find them.

The difference between Douglas Fir and stuff like Southern Yellow Pine in practice is fairly minimal. Even if the wood "feels" wetter or dryer, it's all going to move. Any dimensional lumber will have a good chance to exhibit checking, curling, and splitting. Pick your pieces carefully at the store, and either build it quickly with some heavy duty fasteners, or let the boards acclimate first and re-mill them.

Further Reading about wood codes on lumber: http://www.thisoldhouse.com/toh/article/0,,1639800,00.html