With this method you can kill two birds with one stone, and possibly get a third achievement early.
Go to any festival site and search the auction house for the
1972 Reliant Supervan III. Most seem to sell for 5,000 to 20,000 credits. Buy as many as you need for as little as possible. Or, only buy one to make sure this works for you. If this is your first purchase you'll get the Auction Fever achievement.
Now that you have the cars, list them at the lowest prices available.
Starting bid : 4,000
Buyout: 19,000
Time: 1 Hour
Since these appear in the rare car search as well as the basic search, and there's no way to buy them from the store, they sell pretty regularly. You may not make a profit this way, but it keeps the cost fairly low. Just be aware there are a lot of them being sold so bids come at the final minutes.
If you haven't found this barn find yet, you can also take the opportunity to upgrade one and earn the Fly Like a Regal achievement.