You should join an aquarium society in your area. They trade fish all the time and you will find someone that will take a common pleco. A person posted a common pleco for free and it was gone within 2 hours.
I have picked up 5 rainbow cichlids, java moss, 5 dwarf indian mudskippers in the last 2 weeks and they were all free.
Another reason you should join besides talking and meeting with other fish people and learning...they will offer items for to fellow members before it goes to craiglist etc...
I picked up two 125 tanks for $100 each with and emperor 400 for free.