There are many places to find themes; the list that follows is not exhaustive. A Google search for "Magento theme" will reveal more suppliers. The following list is ordered by number of themes available; the first in the list has the most themes.
- Templatemonster (http://www.templatemonster.com/magento-themes.php): Here you will find a wide range of wildly differing themes. Unfortunately the quality of the themes can differ widely too, so make sure to at least use the live preview function.
- Themeforest (http://themeforest.net/category/ecommerce/magento): This site has a good system in place that allows you to recognize the good themes (based on number of downloads and ratings), and themes are priced relatively cheaply.
- AheadWorks (https://ecommerce.aheadworks.com/magento-themes-and-templates.html): This site is a quality extension builder that also sells some decent Magento themes.
- Polarthemes (http://www.polarthemes.com): Here, only a couple of themes are available, but their work is high on quality. Their templates are also highly suitable for further enhancements by you.
On Magento's website a list of Magento partners sorted by country can be found. These are divided between Gold, Silver, and Bronze partners. These designations currently do not say a lot. Magento partners have sufficient Magento project experience, but the Gold, Silver, and Bronze designations have been mostly decided by how much money a Magento partner is willing to pay to get the partnership certificate.
Magento is planning to make the demands for the different partnership programs depend on, for instance, the number of Enterprise projects delivered and the number of certified developers employed by the partner.
This does not take away the fact that you can find experienced Magento partners in Magento's partner list. You can find the list at the following link:
http://www.magentocommerce.com/partners/find/solution-partners/
Of course there are many companies who are not on Magento's partner list, but are still perfectly able to perform Magneto work for you. A Google search for "Magento project" or "Magento store" will yield ads and search results for multiple suppliers.
Another possibility is hiring a freelancer. The advantage of a freelancer is that you will be talking directly to the person building the theme. Finding a good freelancer is considerably harder than finding an Internet agency that can make a Magento theme though.
If you commission a freelancer or agency to develop for you, we recommend that you ask for references and check up on them.