Welcome to the madness! You're going to find that the 4106 is a great coach, simple yet rugged. What's the VIN? PD4106-XXXX?
Also, since you're new, please take a moment to edit your profile to include a signature line - similar to mine below - with at least your first name, your home city/state, and your coach make & model. That way we can better help you source parts and service. No sense referring you to somebody in CA if you live in FL, right?
Speaking of parts/service, you need to write down and/or memorize this number: 1-888-262-2434. That's the toll-free number to Luke at US Coach in NJ. Strong supporter of the our little busnut community, he has probably forgotten more about GMC buses than all the combined knowledge on these bulletin boards. Reasonably priced parts, ships the same day when you pay with plastic money. Support those who support us!
Be sure to also check out the BBS over at http://wwww.busnut.com
. Not quite as much activity as there is here, but has a great archive that goes way, way back. Oh, and there's also a gmc busnuts group on Yahoo, too: http://autos.groups.yahoo.com/group/gmc-busnuts/
After all that, to answer your original question, you can contact the folk at IBP in Apopka, FL. They've been making replacement panels for GMC and others for quite awhile. Here's their website: http://www.ibpindustries.com/
Personally, I love the look of a 4106 as they came from the factory. Putting front/rear caps and blanking out the windows just, for me anyway, destroys the classic design of the coach. Quite a few folk have figured out how to do what needs to be done without blanking the windows - get creative! Motion Windows, a division of Peninsula Glass in Vancouver, WA, can and does make replacement side windows, both fixed and with sliders, for the coach, so that's an option, too. http://www.motionwindows.com/
Oh, and FYI, the rear window glass is now made from unobtainium, and has been for over 10 years now. Those who have a broken one, especially the center section, have come up with several different ideas, from simply using sheet aluminum or steel or having one made from Lexan at a plastics shop, using the old glass as a mold. The worst, IMHO, is an R&M cap - too much of a stick n staple look.
Anyway, welcome aboard and feel free to ask lots of questions, you'll get lots of help, along with some crazy banter sometimes!
FWIW & HTH. . .