There are many places to buy WordPress themes. However, some are better than others. Here are the places I highly recommend.
I'm a big fan of ElegantThemes. For a few reasons. The value is simply outstanding. $39 for access to 86 themes. That's $39 a year, mind, but still. New themes are added regularly, too, for which there is no extra charge. Of course, the low price per theme ($0.48) isn't what makes the offer such good value. It's the fact the themes look great, come with a bunch of handy features (shortcodes, page templates, etc) and the support is excellent.
Support is provided via a forum. You post your question and within a few hours or so, typically, you'll have your answer. Multiple staff man the forum and in my experience know their stuff. Also, they are available for custom work if you want something outside the scope of regular support done.
Everybody knows and loves ThemeForest! I say this slightly tongue in cheek, because not everyone loves ThemeForest. Some theme developers, for example, complain about the quality of the code in some themes. However, in response to such criticism, as of September 2013, ThemeForest has issued new theme submission requirements, which should go a long way to addressing any concerns. In my experience, if you stick to the themes with a decent rating (i.e. 3.5 or more) and/or by elite authors, you can't go wrong. It's also worth checking the theme comments thread to see what customers who have bought the theme are saying about it. If they are asking for help, is the author replying, and how quickly?
ThemeForest may be a marketplace, but there is a rigorous theme review process in place. I know experienced designers who've submitted themes that have been rejected for no other reason than the marketplace has enough themes of that type already. As the largest theme marketplace online, ThemeForest can afford to be picky.
The pricing on ThemeForest, while nowhere near ElegantThemes levels, is still very reasonable. Expect to pay between $35 and $60 a theme, depending on the functionality. Speaking of which, you'll find some awesome app themes on ThemeForest. In fact, you'll find every type of theme under the sun! New themes are added to the marketplace every day.
I love StudioPress. There, I said it. The most expensive option here, but also, for certain kinds of website, the best. If you're looking for a theme with special out-of-the-box functionality, like a dating theme or a directory theme, you should look elsewhere. But if you're designing a website for a small business, individual with a portfolio, or you're a blogger, you're bound to find something you like.
The framework is also really easy to work with, fast, secure, and provides all the SEO options you could wish for. The designs blow my mind too. They're just so clean.
You can grab every StudioPress theme currently available, as well as every future theme, for $349.95. That's a saving of $900, which isn't bad. So not bad, actually, that I went and bought it. And I'm glad I did. It's probably worth mentioning at this point that I've bought several themes from ThemesForest and I'm also a customer of ElegantThemes. I've used them all. Any questions, feel free to ask in the comments. Cheers!