Much research, tests and reviews of vintage lenses, especially for use on digital DSLR cameras can be found on the comprehensive site of Vintage lenses for video
My favourite Youtube channels with lots of reviews of vintage cameras and lenses:
The Art of Photography: Ted Forbes and his reviews with videos about photography.
Zenography: Thoughts and reviews of cameras and lenses both old and new.
Reliable vintage camera shops (on ebay, no affiliation!):
Petrakla camera’s (Holland, all vintage cameras with CLA!)
Analog Lounge (Germany)
DeWit Cameras (Holland)
And if you are patient, it will be rewarded on catawiki, every week hundreds of vintage camera’s and stuff.
And if you ever need a manual to one of your old camera’s, the most fantastic site belongs to Mike at https://www.butkus.org/chinon/ My compliments for this historic database of all camera’s.