In the school bus conversion community, GMAC is widely regarded as being the only insurance company to cover a school bus conversion. I believe they only offer liability, but at least it is some coverage, and will get you legal.
Some people have had luck getting State Farm and Nationwide to cover them. Every report of that that I have seen has been a person that was a customer of theirs for years, and their local agent got the policy for them. I know others have been declined for bus conversion insurance from those same companies.
Progressive will issue a policy via phone or via the internet, and then cancel it about 2 weeks later as they do not provide school bus conversion policies.
What kind of bus did you get? I look forward to seeing your progress as you convert it.
I hope this information has been helpful,
jim