eBay is a great source of auto parts. I've found things there I couldn't find elsewhere. Also Hemming's is a great source for locating collector car parts.
Keith, I punched in "1979 Ranchero" on ebay and found quite alot of parts. No bumper or tailgate but the thing I do and have luck with is finding sellers that are listing parts for the same car (like rear tail lights, doors) and send him a polite question," Would you have or do you know someone that has a blah blah blah. Most will always reply with a yes or sorry no. Some people have stuff they don't think is worth selling or think to big to ship and don't list it. Some like ebay, some don't, check feedback scores and go with your instinct.