A double boiler is an expensive solution to a homemade bain marie.
Just get a saucepan and a bowl that is slightly larger than the saucepan in diameter. You don't want it to be able to touch the bottom of the pan.
Next half fill the saucepan with boiling water and put the bowl into the saucepan. Experience and knowledge of what works with what will come into play here, but basically the bowl needs to be in the water for a couple of inches, but not touching the base of the saucepan. Now lift the bowl up slightly and wedge something like a spare wooden spoon into the gap between the side of the bowl and the saucepan - to let hot air escape and stop the bowl from jumping around. Do this on the side away from you.
You are now ready to melt your chocolate. You control the heat of the chocolate by the heat of the surrounding water, so a slow simmer is usually all you need.
If the chocolate is melting too fast, lift the pan off the stove or add cold water to the saucepan making sure not to get any in the chocolate (last thing you want). Melt chocolate (any type) slowly and you can't go wrong.
One cheap solution with equipment you will have to hand and no money spent.