We used Progressive and got ours insured for $200 for the year. (It would have been only $175 but my husband had a recent ticket.)
My state (Montana) does not require TTs to be insured but it really gives us a peace of mind.
Oh, here's some wisdom I heard: Don't insure your TT with your car's insurance. If you have a claim with your TT, they may raise the rate on your car's insurance as well.