My info is old, but since no one else has replied, I'll give it a shot. In 2006, I rode from the USA through Mexico, Guatemala, Honduras, Nicaragua, Costa Rica, Panama, Colombia, Ecuador, Peru, Bolivia Argentina, and Chile.
I had been to Mexico several times before and organized liability insurance, which is all I ever carried, through
www.mexadventure.com. Mexico does not recognize insurance from foreign countries, only Mexican companies, and I think there are only a few of those. There are many resellers, the above website being one.
Nicaragua and Costa Rica required purchase of insurance as part of the border crossing procedure, and it was easy and cheap to buy.
I air freighted my bike from Panama to Colombia, and had to buy insurance before I could get the bike out of customs. It was reasonably priced, but I had to get a local to direct me to somewhere I could buy it. I think Colombia was the only country I was asked to show proof of insurance once I got past the border area.
In Argentina, I bought a policy that I was told was good for the MercoSur countries. I had my doubts, but was never asked to show it, so who knows.
The other countries, I had no insurance either because it was not required, not available, or both.