I am 70 and always use Squaremouth.com, which always offers me lots of choices; so many that comparing them is a bit difficult. They allow you to pick policies to compare and, once you get the hang of it, the compare feature is very useful.
I just used Squaremouth for some friends in their 80's and Squaremouth also had many choices for them. Travel insurance is definitely available for seniors although, understandably, it gets more expensive as you get older.
Be careful about reviews - most people never make a claim, of course, and so most reviewers know nothing about how well the claims process works with the particular company. Look for reviews by reviewers who have made claims. The others often just talk about how easy it was to buy the policy, which isn't really helpful.
Many of the companies insure existing conditions, but only if you buy the policy shortly (it looks like 2 - 4 weeks depending on the company) after you first spend any money on the trip; e.g. when you order your airline tickets, or whatever. This is probably too late to help you on this trip, but it might be useful to know in the future.