MainStage has a massive treasure trove of very musical, usable patches. Unfortunately, sifting through tens of thousands of patches for the right sound can be daunting. Here's a quick way of locating the file you're looking for quickly:
1. Open MainStage. Select your project.
2. Hit command-F. This brings up a dialog window that allows you to search for content instantly.
I stumbled upon this handy little feature last year when I was goofing around, and it's saved me literally dozens of hours of searching. If you'd like to get a free download of my favorite patches that I've found, give me an email and I'll send them to you. Or you could save yourself the time, and get some patches from my online store. Good luck!