www.themeforest.net under the HTML templates category. Also CSShub is a good resource. You will want to select a template that fits your vision and can accommodate your app's structure. Probably want to personalize it a bit too, making minor changes.
In the past i have blended a few themeforest templates together, taking footer from one, layout from another. It gives you a quick head-start.
Best of luck.
I've been looking for a SaaS styled template for my WordPress website, and unfortunately I haven't found a lot of choices.
As Franky mentioned, Theme Forest has a few. Here's a direct link to the Theme Forest SaaS templates. It looks like there are about 6 templates that fit your criteria - probably less when you look at them in detail.
I also found this question on the WordPress StackExchange site that lists the best sites for WordPress themes. It's worth a look even if you're not looking for WP themes, because not all the sites listed are WP specific (Theme Forest for example).
If you happen to find good SaaS templates, please come back and let us know.
If you are open to modify some HTML and PHP code as well as some graphics, you may want to consider free themes such as:
Templates are horrible. The HTML/CSS can be quite bad. I've seen horrible work out of well-known places like TemplateMonster. Those sites don't really vet the work they resell.
Also, depending on your target audience, they may notice it is a template... and "the interwebs" can turn it into a minor PR disaster.
Find and work with local talent. Not only do you get what you need, you'll likely need to do updates/tweaks and you will have started a hopefully good relationship with a good individual or company.