Ebay is a good start. Search in Cycling for "wool". Another place to find secondhand 70's wool is either in second hand clothing stores, or, believe it or not, in shops that specialise in rave gear. I got a couple from a raver shop and they were only 25 bucks. It is tough to find big sizes though, I have to warn you.
Companies that have done or still do wool tops are Salsa, Wild Woolies, Woolistic, Alessandro, Smartwool, N-Zone, Icebreaker, Kucharik, Woolywarm, Ibexwear etc. I'm sure there are many others, but I can't remember them right now.