Either a fridge or freezer will work, and they both have their own advantages and disadvantages. You need to make sure that your fermenter or fermenters will fit inside, and that it cools. It doesn't matter if the thermostat doesn't function properly, as it can be bypassed. The first item I found was a stand up freezer listed for free. Upon inspection, I found that is was not frost free, meaning the cooling coils ran through the shelves, which means they can't be removed to make room for the fermenters. Make sure to look for a frost free frezer if it's an upright model. Chest freezers work very well, but can be harder to find. They have the advantage of keeping constant temperature better when you need to open the door, and most often don't need any modifications other than adding a temperature controller. A regular refrigerator also works well, and the freezer portion can be used to store hops, cold mugs, and other items.
I didn't want to spend much money, and because of it's unique use, I could live with a lot of problems that might reduce the price. I kept checking Craigslist until I found the prefect thing. It was very dirty inside and out, missing the decorative plastic grill on the bottom, had a broken shelf support, was missing the crisper drawers, and was only $20. Since it was going to live in my workshop, I didn't care much how it looked, and I'm happy to do a little cleaning to save some money. I also didn't care about the shelf or drawer issues, as I planned on removing them.