Post your location, a link to your local Craigslist, and perhaps someone will give you a suggestion. Frame size? 54ish?
20 lbs is a tough target. From the building perspective, you'll need a super-light frame, lightweight bottom bracket and crankest, lightweight wheels. And then can't ignore the weight of anything else. And, it all comes at a cost.
Keep in mind, however, the best deals are snapped up quickly without time for a lengthy online debate.