Shutterstock sometimes has Instagram or website links in the bio, but that's probably your only option.
It's very frustrating, they don't have any links. Did the sites have messaging before? I really feel like at least some did at one point.
If their work is on Dreamstime, you can send a comment from one of their images to the contributor. iStock used to let you site mail contributors, but that was long ago.
I can often Google someone to find a twitter account or some other presence outside of an agency (although you need to be sure it's the same person, but many contributors promote their work via social media). Or do a Google image search on one of their images to see if anything useful turns up.
If the agencies would leave metadata in the images, it'd be easier to track artists down, but that is obviously why most of them delete all of it (and Adobe Stock keeps most of it, but not the URL link for copyright information)