You can try some official manga platforms that offer free sections or limited-time free reads. For example, some publishers might have promotions on their own websites.
First off, you should have the right tools. Then, pay attention to details like cleaning up any noise or artifacts. Also, make sure the contrast and brightness are balanced to bring out the best in the scans. It takes some practice and patience.
I'd suggest checking out some popular online manga forums. Sometimes, users share links there. But again, make sure it's legal and respects copyright laws.