Are you a USAA member? I bought some through a USAA partner; found it on their web site.
We only bought it once (when the Iraq war/conflict/invasion...whatever you want to call it... started a few months before our vacation). We got it through Travelguard. We didn't have to actually "use" it so I can't speak to that end of it, but I think they're fairly well-regarded.
I made the mistake of buying it for medical cancellation only. I will now only buy the kind that covers every conceivable problem. Very expensive misunderstanding on my part. Have a good trip!
You want Cancel for Any Reason insurance. And, yes, you generally have to buy it at booking or very soon after ( within 7-10 days)
We usually purchase it with our airline tickets - via the option presented when purchasing them. The policy then covers the amount of the ticket purchase price for trip cancellation plus medical and other coverages. You can later add to the coverage level for trip cancellation (such as for non-refundable or committed lodging payments, etc.) I was able to do this fairly close to the date of my last trip. We usually just get it for very costly trips or trips that are outside the US, since medical coverage benefits are included and our health insurance only covers within the US.
If you purchase the insurance that you can get through the airlines, read the fine print! After we purchased it, someone mentioned this to me and by reading the fine print discovered that we would not have been covered due to the reason we were purchasing the insurance in the first place.
I usually purchase it through Travelex. I don't insure the airline tickets since (in the past...Not sure what policies are now) they can be exchanged for a fee and for no fee for a documented medical reason. I only cover costs that cannot be reimbursed in any other way..so, hotel reservations that can be cancelled 24 hours in advance, I wouldn't cover; an apartment rental with strict cancellation, I would.
I have definitely purchased insurance more than 1-2 weeks after booking a trip. Just read the fine print carefully, especially if there is a recent health diagnosis. Also, be certain that your health insurance policy will cover you at your destination. If not, you might want to purchase coverage.
I checked out Travelguard. Some of the potential cancellation reasons are covered only if you buy the insurance within 15 days of the first trip-related booking you paid for (airfare, hotel, etc.) I suppose you'd have to provide documentation if you actually needed to cash in on the insurance. But most of the cancellation reasons don't have the 15-day restriction.
Can anyone recommend a trip insurance company to protect against unanticipated last-minute cancellation? And is there usually some requirement for how far in advance it needs to be purchased?