Spare wheel, what to ask for and what they are worth

[dmjlambert]

I have a 1969 long bed pickup that I'm converting to stock height 5 lug 71-72 setup. I have these wheels (picture). I understand they are 1975 and later truck ralley wheels. I think they are 15x6.5 and they have P275/60R15 tires.
I am looking for a 5th wheel that I can use as a spare.
Are plain wheels that would have originally came on 71-72 and later trucks common and easy to find? What do I ask for if posting a want to buy ad, what do they look like, what are typical widths and tire sizes to use, and how much should they be worth?