If you have a buyers list, find out what type of houses they are looking for, the locations and price range.
Depending on the area you are looking in, finding great wholesale deals takes time , effort, and usually some marketing budget. You can drive for dollars and knock on doors. The only cost is gas, but time consuming. This is how we found our first deal when we where starting.
I have never been good at Facebook, but I understand you can find deals there as well.
Bandit signs are usually only efficient if you put up hundreds and they don't get removed from the local police or you don't get a fine. But you are probably not going to see a significant number of good calls.
The MLS may have some properties you are looking for, if you have access. You may be able to partner with a realtor interested in what you are doing.
Letters work, again this is not cheep. It use to average us 1000 letters a month for 6 months to get 1 house.
There is no perfect way, but if you are consistent you will find the deals you are looking for.