Is there a list of Roots-based themes that are available publicly? I love the idea behind Roots and am looking for a finished theme to buy rather than styling up from the developer theme …
There isn’t right now. I know there’s some Roots based themes on sites like Themeforest but they can be difficult to find. The plan is that we’ll be adding “official” Roots themes to our site at some point (hopefully sooner rather than later).
How can I find them on Themeforest? Know any names? 
Or WP Codex?
Not off the top of my head, but I can tell you there’s yet to be one that I’ve come across that I would recommend purchasing since they’re not modifying/using Roots optimally.