It's happened to many of us: you find yourself stuck in an internal battle on whether or not to rescue a sick fish or one who has been sitting in a cup for a long time and has yet to be taken home. Some of us can find it nearly impossible to walk away when you know that the fish will probably go to an abusive home if it ends up going to one at all. Also, it's hard to pass up that really amazing fish you find that you never intended on buying. The resulting situation is often that the rescuer or purchaser does not already have a habitat set up to put the fish in when they get home.
I found myself in this very dilemma when I rescued a fish with tailrot from Wal-mart who I noticed had been in his cup for over 10 days. I had very little money but I felt wrong for just leaving him there when I knew I could give him the best home possible. I already have another betta, so food and conditioned water weren't a problem. What I ended up doing was purchasing a very large fish bowl from a thrift store (which is fine until I can afford a more permanent home since the fish is very tiny), and filling it with marbles I had as a child along with some 99 cent aquarium plants I found at a store. I went to the dollar store to buy a clip-on reading LED light (one that does not produce head) to give him a little light. Next, I created a makeshift lid by taking a plastic lid from a food container, rinsing it, and cutting holes for feeding and air, and the heater cord. The most expensive item was a bowl heater, but the entire setup including the fish was 15 dollars at most.
It is possible to give every fish a safe and comfortable environment, temporary or not, for little cost with some brainpower and dedication. If you have an emergency situation and need some pointers on how to set up a quick habitat, ask here! If you have some suggestions or personal examples about how you have created makeshift or rescue tanks please share them! :)