I've been using American Collectors Insurance for my FC since 2003. I believe they cover all US states; I've used them when I lived in NJ and now in MD. Here's the website for more info:
http://www.americancollectors.com/
The gist of it is they sell "agreed value" policies for auto hobbyists & collectors on a case-by-case basis with lots of terms & conditions. If you & your car qualify and you can live within the terms & conditions they stipulate it's a great deal. In my case, full liability coverage and agreed value collision & comprehensive on the car costs me a little over $200 per year. The only catch is they won't cover any competitive racing events or driver school/track days, so if you do that sort of stuff, consider buying the supplemental insurance coverage that is typically offered by the organization that hosts the track event to CYA.
Check with them for all the current requirements, but here's what I recall they were when I first applied for a policy with them:
1. Spotless driving record, plus a minimum driver age requirement, think it was 35 (no problem for me, I was in my 40's when I applied)
2. Car must be garaged, and you need more registered vehicles in your household than there are drivers - in other words, the insured car can't be your daily driver. So if there are 2 licensed drivers in your home, you need 3 cars - 2 daily drivers plus your toy.
3. No other licensed drivers under a minimum age living in the household - I'm a bit worried about this one, since my oldest son will get his license this year.
4. They don't impose mileage limits, but they will ask you for a ODO reading each year when you renew, and if you racked up more miles than your usual (based on previous years) expect some questions. The first year I did the DGRR trip, they asked me to explain how I racked up an extra 2000 miles or so. They were OK with the explanation and didn't raise my rates.
5. Depending on the type of policy you apply for (collector vehicle, modified, street rod, etc.), you'll have to provide written justification plus pictures for them to consider your application and your proposed car value. If they accept your application, they will have you take the car to a local shop for photos, and make their own independent appraisal for the agreed value coverage.
BTW, you don't need historic/classic plates to apply for this insurance, and I had regular plates when I applied. Just this year I switched to historic plates, since in MD that makes registration cheaper ($52 vs $128 every 2 years), and I can now dodge the tailpipe sniff test.