For a professional design look and feel, I get my templates from two sources. One is Yootheme.com and the other is Shape5.com. For what it's worth, these two companies have the best templates for Joomla 1.5.

It costs $45 to get templates at either of these companies and I use both. I use these templates for my own sites and for my client's sites. They are cheap and beautiful and that's what I need.

I prefer Yootheme over Shape5 because Yootheme's themes always work where I've had some issues with Shape5.

Anyway, if you want a professional theme, you can get them at those two places. They are easy to install and integrate into Joomla!. They are actually made specifically for Joomla! and the installs are one-click installs.