There was a member on here, greentea, that did a rescue. They put up fliers saying to drop off unwanted bettas in her mailbox - not sure where they live but I am assuming this was only in the summertime. Sadly I haven't seen them online in a while.
I can't have a very big rescue because I am confined to a 10 x 10 foot room. I can only have about 1-2 tanks right now.
It wouldn't have to be "big". Bettas really don't require much space as compared to say a dog or cat. You can keep them in 1 or 2 gallon tanks assuming you do the work that a small tank requires.
All of my bettas were "rescues" from petco - although I don't offer them up for adoption.
I had 23 when I lived in Alaska and my entire apt was a mere 280 sq. ft - all were in 3 gallons or larger