I used an old fridge. Try to find one that has as little plastic as possible on the inside but old ones that where all metal are hard to find.
Line the inside back, sides , floor, roof and door with metal. For the sides use house siding so that the ribs are used to slide old oven grates into. You may have to wire two together to make them fit.
Cut a hole in the top aprox 4 to 6 round and install a tight fitting lid that can be opened or closed to allow smoke to escape.
I use a one burner electric hot plate as heat source placed in the bottom. Place a small cast iron frying pan on the hot plate, I use a metal coffee can with holes drilled in to it to hold my wood, place upside down in fry pan and close door. Smoke will start in a few minutes depending on heat level. By putting wood in the can you prevent flare ups and possible burning of food.
Install a thermometor thru a hole drilled in the side. I use my deep fryer one. By turning the burner on and off from outside the smoker you can control the heat. Depending on what you are smoking heat level and time will vary. I find one can of wood lasts about one hour and needs to be re-filled.
I use willow as wood but oak, maple etc will work well too.
Cost for all this will be next to nothing but it does take a bit of work. Hot plate is about $30.00