I've used the following sources for vintage Maserati parts, and found all three to be very helpful:
1. Jacques Pozzo di Borgo at
www.maseratisource.com (ph 909-336-9389) (Southern CA). Stocks new repro parts as well as lots of uncatalogued used parts.
2. Kyle Fleming (ph 757-499-5170, fax 757-499-8194) (VA). Stocks lots of uncatalogued used parts. Has been dismantling Maseratis for decades, vast knowledge but doesn't do the internet.
3. MIE at
www.maseratinet.com (ph 253-833-2598, fax 253-735-0946 (Seattle, WA). World's largest supplier of vintage Maserati parts.
Haven't personally used
www.reoriginals.com, but know people who have. Prices tend to be breath-taking, but sometimes they are the only source for exotic or obscure items. For that reason often seen as the vendor of last resort.
As for finding a windshield, your chances of finding a decent used are reasonably good, since many QP1s were sacrificed over the years for their mechanical bits, not their body parts. You might try posting a Wanted note on the Yahoo board (subscription required) at
Yahoo Maserati GT Group
If you're ever in Boise, Idaho, I'd be happy to trade you a good used QP1 radiator and a rough rear bumper straight across for a six-pack of New Belgium Fat Tire. They came with a package-deal for some Sebring parts I bought a few years ago, and have been taking up space in my shop ever since.
Cheers!
Doug in Boise, Idaho USA
1963 Sebring Series 1 (current)
1960 Vignale 3500 Spider (former)