Search, Search and Search some more. Look everywhere, local craigslist ads, local papers, back yards, ask friends, and of course this sight.
There are still a few broncos that can be had for less than 20K. Dont know what your budget is but the days of picking up a driver for 5K are over. You can get a running rust bucket for around 10K maybe.
I have purchased broncos from all the places I listed, and one of the best ones I ever got wasnt even for sale. One key thing is to have the money ready to go. A deal may pop up and if you can be the first to show up with cash in hand you will have the best chance of getting one.