So... Southern Cal, where the OP is from, is where I worked for 10 yrs as an EMT.

When I got my EMT, back in 96, no EMS affiliation was required (in LA County). However, the state is ack-bassward. Every county does their own EMS cert's, or you can go through the state Fire Marshall course. so, concievably some counties would want you with an EMS agency, others wouldn't. But to my knowledge, none did. All counties offer reciprocity with one another, for a fee. Wildernss FA is tough to find, depending where you live (I think UCLA EMS offers it still).

Nat'l Registry doesn't exist in CA. However, some counties use NREMT as the basis for their tests, such as Orange County, so you may be served by learning that curriculum. LA Co. was a simple "home made" test.

I'd say call up the EMS agency for whatever county you live in, get the scoop on what they require, and where they have courses. Shouldn't be too pricey. If you don't like your options, go to the next closest county and repeat. You've got, what, 5 counties to choose from (LA, Orange, SD, San Bernardino and Riverside). Santa Barbara/Kern County too if you wanted (I heard good things about their tests)