@lucaweb: StudioPress Themes are GPL, which means that they can be freely modified and redistributed; however, such Themes are often targets for unscrupulous persons who make “modifications” such as SEO/spam links, or adding malicious code. It’s mainly for that reason that I hesitate even to download a Theme found through a “free wp Themes” Google search.
As for the support, though: it’s mainly a matter of principle. StudioPress offers paid support for their Themes (which is perfectly acceptable); most here will generally only offer free support for free Themes, since otherwise we’d be doing gratis what another developer gets paid to do.
The other issue, as I mentioned earlier, is that, with paid Themes, we can’t get access to known-good Theme files without paying to download the Theme. With the unscrupulous people mentioned above, it is not merely possible, but rather extremely likely, that the “free” versions of the Theme are obsolete at best, and maliciously modified at worst.
All of this is a long-winded way to say: your best option is likely to pay for the Theme from StudioPress, which will give you access to their Theme support. From everything I’ve heard, StudioPress Themes are great, as is their support. So, it would only be to your advantage to go this route.