Are there any (online) communities for independent researchers?
IGDORE, a large organization for independent scientists, hosts an online forum titled On Science and Academia. This is by far the largest online meeting place for independent scientists to my knowledge. IGDORE membership is not required to participate. I am not a member of IGDORE, and I have seen others with traditional university affiliations participate as well.
I've found the website useful to meet like-minded people, and I believe they would be receptive to requests to collaborate on a particular idea.