Have you tried the dealers parts department or rockauto.com for new parts. The pick-A- Part or regular junk yards are another source for used parts. Or you could have more luck posting this request in the classified section on this site.
Grumpy,
I'll try to do some research for you. I'm parting out a '79 300. I'm pretty sure it has power windows. I just need to check to see if the regulator is the same as a '76.
I'll try to get back to you in a couple of days.
Dallas