Do you also have bronchiectasis or MAC? This is a group of patients with lung issues and may not be able to lend much advice for stiff heart, but provide valuable info on bronchiectasis. I did see a Mayo group for heart and blood issues.

Is your stiff heart from Amyloidosis, the most common cause, I think? Mayo has an amyloidosis clinic and they might suggest a support group. There are also Facebook amyloidosis groups, too.
There are also other amyloidosis clinic around the country. If it’s caused by hemochromatosis or another less common cause, I imagine a general heart group might be able to connect you with others.

Good luck to you. Others might chime in if they know of resources.