Ebay is actually quite good. Bought mine there after looking 6 months everywhere else I could think of. If you are prepared to travel with a tow dolly or trailer to get one, they will go cheap on Ebay depending on location.
Vintage Truck mag has some listed in each bi-monthly issue.
The best thing I've seen is a weekly publication called Truck Roundup, but it can only be found in some gas stations. There are all kinds of good old trucks in S. Dakota I've seen in that publication. Search the internet and you'll find the site. Never pursued it but there is a way to get their ad electronically. This is where I would start if you, and I'd check Ebay daily at the same time.
