How can I get travel insurance on a trip booked in January?

I have looked at a few insurance websites and they all seem to say the insurance must be obtained within 15 days of travel booking. Well, I paid for the trip in January and it’s for late July. I had no idea I’d need to get insurance my mid-February. Does anyone know of any travel insurance companies without this requirement?
Sorry, I forgot to add: I’m in the U.S.A.