What is the best resource for looking up older, 1930'-50's, racers (riders)?

Maybe ask over at Stones cyclery in Alameda? Or Velosport in Berkeley when the owner is there.

The bikes or the riders?

For European, https://www.cyclingarchives.com/index.php or https://homepage.ntlworld.com/veloarchive/index.htm

riders, Thanks

So here's the story. My son-in-law was telling me of a trip to Uruguay to visit his dad's birthplace. He said his grandfather, Leonel Rocca, had raced in the Olympics and had designed the velodrome in Mercedes. A local rider had urged him to take a lap on his bike. As he rode around around, the locals had heard that Leonels grandson was riding the track and they started gathering in the stands, cheering. Saturday, his dad broke out a scrapbook, mostly Spanish, and it had a pretty extensive documentation of his career. It contains newspaper clips, race flyers, certificates, awards, neat stuff. I'm not finding much on the internet on him. They said he coached the Mexican team but I couldn't find anything on that. Anyway, I'll see if someone in thier family wants to add to his bio on the Cycling Archives. If I can get some scans of the flyers I'll post them.
https://www.sports-reference.com/olym...l-rocca-1.html