I've used AIG Travel Guard and Tin Leg. I know there was a 3rd company I used at some point but can't recall who it was. But in all 3 cases, I know a requirement was that I had to buy the insurance within 2 weeks of the initial booking of the trip.
I can't tell from the original post if you just booked but want to get insurance through someone else, or if you booked awhile ago and want to add insurance now that it's getting closer.