I am interested in starting to dive side mount. . . . I never took a Drysuit class and have been diving them for years. I learned by reading and finding a someone accomplished at diving dry. Kind of like an apprenticeship. Couldn't sidemount be learned the same way?
Your post actually asks two, related, but different questions: do you need certification (the title), and do you need to take a class (tyhe body of the post). The answer to both is 'Maybe, maybe not. It depends.'
You can learn just about anything without taking a class. Go to YouTuibe, and there are instructional videos that will teach you almost anything (within reason). For that matter, open water diving is pretty easy, you can learn it without a class, and there are plenty of people diving today who have never had a class, nor have any formal 'certification'.
I learned to dive a sidemount configuration without a sidemount class. I didn't even use an instructor, I just asked some questions, observed other people, watched some of Lamar's vidfeos on the DR website, went out and dove a DIY SM configuration, and felt like I actually was able to do so with some skill. Then, I took a class - from someone who finally got tired of my never-ending emails with questions - and realized how incredibly inefficient my previous learning had been.
If you can find someone to dive with, who will be willing to take time to instruct / mentor you, fantastic. Go for it. I doubt very many people are going to ask, 'What class did you take?', unless you look really good, or really bad, in the water. You might find that taking a class prepares you to more readily / effectively / efficiently learn on your own, or you may not. You probably won't be asked to show a 'Sidemount C-card'.
If someone asks me today, 'Can I dive with you so I can learn sidemount?', I might say, 'OK' but I have will probably little no investment in the process. They take what they get, when (and if) I have time. If someone says, 'I want to take a sidemount class with you', I say, 'OK. Here is all the background information, equipment rigging approaches, skills, etc that we are going to cover, here is how we are going to schedule the class, here is what it is going to cost you. And, by the way, after the class, if you have any questions, or need more help, or just want to dive some more, let me know. You purchase a lifetime learning pass when you take the course.'