Single Trip Insurance at www.ihi.com

I added the higher altitude rider to the American Alpine Club insurance.

I think your Aconcagua permit covers any medical you need on the mountain, including the heli evacuation to trail head if needed. Not sure what happens if you have to go to a hospital.

Depends on what you need it for. If it's just for rescue/evacuation then it's included in the price of your permit.
If you want something more than you can buy it from almost any insurance company as a custom option for their regular travel insurance but it's gonna be quite pricey.

I'm going to Ecuador next month and was looking at insurance at insuremytrip.com. There are several medical policies that will cover high altitude with the addition of a rider that costs an additional 20 or 25%. I've used these guys for several trips to the Himalayas.

The price I was coming up with for 32 days in Ecuador was around $156 and I'm 63 so being an old fart probably increases the cost. There were no altitude limits with the hazardous activity rider.