There are a TON of themes in the "Contributions" section of the OS Commerce website. Have you tried that?
You might also try Free osCommerce Templates - osCommerce 2.2, STS, 3 | freeoscommerce.com
They have some free ones, as well as some paid ones, but even the paid ones are cheap - I think I paid like $9 for a theme for one of my websites and it looks pretty good.
There are several other sites as well.
But if you want something 100% unique, you will have to pay someone. Those can range from $80 for the simple stuff, up to $2,500 or even more, depending on how complicated it is.
The free ones are good to start with, however. They look nice and you can play areound with them and learn more about how they work. That will give you the knowledge to make sure you get exactly what you need when purchasing a template.
I hope this helps!
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