As far as where to look, I'd start with this list of frameworks, and email them directly to see if they recommend a specific theme for your service niche:
There are many factors to consider when purchasing a premium theme:
1.) Is there an active support community? This should be at the forefront of your mind, as there's nothing worse than needing support only to find out that it's an outdated theme that doesn't get regular updates. Take a look at the changelog and see how often updates are pushed. Ask them about support turnaround time.
2.) Speed of theme - You don't want a theme that is bloated and will slow down your site.
3.) Aesthetics - Spend some time taking a look at demos of their theme. See if there's some sort of 'showcase' that shows other sites using that theme. Peruse those sites
4.) UX/UI - Is the theme intuitive enough to navigate around? Can you add your own custom CSS if necessary? Maybe you're not tech savvy, but if you decide to have customization done, you can always outsource having CSS written for you, which you can then add in the appropriate section within the theme's settings.
5.) Framework - Is the theme that you're considering part of a larger framework? For example, Themeforest is a framework that provides many different themes.
6.) Reviews - Check the reviews/testimonials on the theme.
Let me know if you have any questions :)