Which site is best to purchase a theme?

I suggest you to use freelancing website to do this work. You can post as project in any of the below website.

Fiverr
Freelancer
Upwork

There are many Photoshop experts who will design themes for you. You can also check the reputation of workers in their profile.

If you are being specific to say wordpress themes, then you should check out Themeforest. If you are interested in an entire HTML based website, then checkout a marketplace called codecanyon.

There are some other marketplaces where you can either hire or use the existing designs. You should definitely checkout freelancer and upwork. Most of the people over there are good designers who can be useful.

These are the best stores to buy themes for websites. Their support is very good.

1. Theme Forest.
2. Elegant Themes.
3. My Theme Shop.

Here are a few good options for buying themes -

1. ThemeForest
They are the best when it comes to themes -premium themes rather. They have themes for just about any genre right from personal blogs to corporate websites. They also have competitive pricing ranging from mid level to high end.

2. WooThemes
They have a wide variety of themes for your needs. Pricing too is on the lower side.

3. Elegant Themes
This is one of marketplaces for themes wherein you can dig deeper into finer details. They have a good customer care which helps you in everyone of your needs. They also offer annual subscription.

Other good options include
1. MOJO themes.
2. Creative market.
3. Themify