I'm in a similar boat, I'm great with desktops but I have no true networking experience. For me, I had to bite the bullet and go to school. As far as schools that offer Network Admin classes only, those are expensive and don't give you anything but a participation certificate

They prepare you to take tests only, but I guess if you want the paper certification without the rest of an education, it's probably not the worst thing you could do. Those schools are easy to find, I've seen plenty at job fairs. I would start in your local paper if you live within 60 miles of a major city. The trick in your case I guess would be to finish without books......