Well, unless someone here can point you to a site/designer that has that sort of thing already setup, your best bet is to make yourself a list of sites and contact them, proposing some sort of deal.
Do you actually want to get involved in the design part of things with your customers? If not, it may be simpler to just point them to a number of free and paid templates sites. In any case, they should be good quality ones.
You can get free template from wordpress and even the paid template is worth. Everyone is put a thumb up for wordpress template that why there are so much people use wordpress to use for website purpose.
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Yes as others have stated there are too many to state here, google is your friend. It also depends on what type of template you want for which cms etc. Joomla, wordpress, etc. www.websitetemplatesonline.com is one