Sometimes there’s a tool online that fits the niche. Andrew’s Aspect Ratio Calculator has been that tool for me.
Elegant Themes
Though not a “marketplace”, such as ThemeForest, with multiple theme authors, Elegant Themes is well established and worth checking out.
With access to over 75 themes for $39/yr, a growing feature list and eye for design, they are worth checking out.
For the all-inclusive price of $39, you are only paying $.50 per theme!
You are also free to use their themes to build websites for your clients.
Lots to learn as you get started – $39 is hard to beat. Check out their Gallery and see if anything strikes your fancy.