I'm getting to the point where I'm transporting a lot of expensive equipment to and from job sites and I'm looking to mitigate my risk both at home,in the car, and at the job site.

Is there affordable theft insurance available for roughly $12,500 in equipment? Which I take to jobs.

Would it be part of home owner/apartment insurance? I'd assume home owners wouldn't cover the car.

Where do I look for it? recommendations?

Not sure how it works in the States but up here in Canada, it's not home insurance nor car insurance but specific BUSINESS EQUIPMENT insurance and it's NOT cheap, especially if you want to leave your home/office/home office with it.

You should try find a local insurance agent/broker who can put together/find custom policies for you. Preferably someone who understands your business. I'd go with a person, not a company. You want personal attention here. For the production world, you probably should have a full-blown DICE policy. For just equipment, you could probably get a separate floater. If you're worried about things being stolen from your car, you probably need to add that to your auto policy (and make sure it covers other vehicles, not just yours). Anyway, not an expert, but I really recommend the agent route. Avoid the online places, they're way over-priced.

I tried to do this awhile ago, through state farm. At first it was part of my renter's policy, but then they decided there was too much risk, and wanted to charge separately.

You'll have to forgive me 'cause it was a few years ago that I last discussed this, but if I recall it was $500 or so per year for around $15k worth of gear.