Craigslist,Kijiji,bargain finders and networking.Once a person develops a reputation as a vintage bike collector the bikes will tend to come to you.Couple of examples that have come my way:a seven fin CZ 125 - the guy said his wife told him to bring it to the dump.He was the original owner.The CZ fired up with a carb cleaning and the price was a case of beer.
1980 Gilera 125 MX - my brother found the bike abandoned in an orchard.
1981 Wheelsmith Maico 250 - a friend found the bike.$250.
In the case of the Zundapp I responded to an ad for a Montesa Cota on Craigslist.Turns out the guy had two Cotas,the Zundapp (free) and a basket case CZ 250 MX ($25).
Once I was driving to town and I saw the cylinder head of a Ducati 250 sticking out of a garbage can (sharp eyes!).Next garbage day the rest of the Ducati 250 Scrambler was parked next to the garbage can.A friend found me a 1975 Sachs 175 (first generation 1751 5A) at the scrap metal dealer.
I know the locations of more than a few bikes - (2) CZ 400s and a Can Am 350 ASE for example.You have to be careful or things can get out of control!