I buy the travel insurance whenever I book the first item of my trip, so that pre-existing conditions are covered. You can always add dollar amounts later.
I have been using Allianz because they seem reasonable and are usually the default for many airline bookings.
Many prople go to Insuremytrip to compare plans and costs but I found them to be competitive for what I want. Their call center is very helpful about adding/changing things. I had my 83 year old mother make sure she has "cancel for any reason" as it was questionable for a while whether she was going on our latest trip.