I'm by no means an authority on this and , like you, am seeking some help in this matter.
I purchased a template from Template Monster only to find out that it was not compatible with my hosts php version.
It was a complete osCommerce install package so there was little possibility of uploading just the necessary files to my existing and up-to-date osCommerce install.
I would hope someone could tell me if there are "add- on" templates and where we might find/buy them.
Templates seem to be one of the weak spots of oscommerce. Unlike Joomla which has a module, component and template installer built into the admin, osCommerce seems to be an upload /replace approach making template developers create a complete install of the OSC package.
If anyone can lend their insights it would be much appreciated.