Look at www.insuremytrip.com and www.squaremouth.com Most policies will cover pre-existing conditions if you purchase within 14 days of initial deposit. After that you rarely will find coverage for pre-existing conditions without some sort of look-back period.
My travel agent signed us up for "cancel for any reason" insurance. We can cancel up to 15 minutes before the flight for any reason with 100% return of our money (less the cost of the insurance). It also covers medical. It was $100 per person. I'm not sure who the supplier was, but if you haven't booked yet I'd make sure to check into this, as most places require you buy the insurance with so many days of booking. If not, I know insurance companies (AAA for instance) offer travel insurance, but it's of less quality as compared to what you can get through a travel agent.