Yes, it is a quest for that special airplane! It is a slow journey, but the rewards are great!

You have to be of the mindset that it exists, and you're going to find it. If you don't find it right away, that just means you haven't looked in the right places. You have to be persistant. If you only read the ad's once or twice a week, the bargains will be gone before you can act on them. Get a feel for where to look. ASO.com and similar sites cater to the higher end sellers. Barnstormers and other "free" sites don't, and you sometimes have to sift through the chaff to find the wheat! Aerotrader is a good compromise.

There are no less than eight Cessna 150's on Aerotrader and a few dozen on Barnstormers RIGHT NOW for under $20,000! Some maybe are good deals, and some aren't! There are a hundred places to look for planes online! Google "Cessna 150 for sale" or similar.

You will likely not find a $30,000 airplane selling for under $20,000. But then, there are very few stock Cessna 150's that are really worth $30,000 on the open market. That's where many buyers get into trouble. They go in expecting to pay a high end price for what they want (a "New" airplane at a used price) and wind up settling for less airplane at that same higher price (a "used" airplane at a used price). These aren't going to be perfect airplanes. No 22-48 year old airplane is going to be perfect if it's flown regularly. If paint and interior are important to you, you're gonna pay higher prices. Same with IFR capability (often grossly overpriced for what you actually get). The real bargains are good solid airplanes that look like crap (nead paint and interior work) but will fly you anywhere troublefree. A pretty hangar queen is still a hangar queen. Your personal requirements should set the price range!
Change your requirements only when the price range is too high!

The real effort comes into play once you've located a list of candidates in a comfortable price range. Then you have to separate the wheat from the chaff, and often you'll need help in that. That's where the experience on the forum comes in. You've already picked out a plane you're interested in before you come to the forum. You've already picked out the good points, and will likely list those in your post. No point to us telling you what you already know (the good points). That leaves us with the responsibility of making sure you (a friend and member of the club) recognize any bad points. We are the devil's advocates in the buying process. When we think you've found a good deal, we'll tell you so. Just because we pick out the flaws in an airplane you like doesn't mean it's not a good deal for you personally, just that the evidence you present (and that we might find elsewhere) shows that it might not be.