I am looking for a case that would work well as a heat sink. Thinking that something like a car amplifier case? Anyone know where one could find cases like this? Would be using it to house H-Bridges and fill it with thermal conductive epoxy to water proof it....
One of these extruded aluminium ones?
Those commercial cases might work, but you'll note that a car amplifier has many, large fins - this dramatically increases the surface area. Those commercial boxes just have some beauty stripes. Also don't forget the volume calculation: I don't know how much thermally conductive epoxy costs, but even, say, a 1 by 3 by 6 inch box has a fair amount of internal volume.
What you also could do is to salvage the heat sinks from the inside on an old PC power supply. These come with screws and insulators and are pretty big in terms of surface area. You might be able to bolt these to the outside of an otherwise flat box (you want to use gap filler between them) to heat sink it. Alternatively, just screw your high temp components to them and the encapsulate over them with your epoxy.
If you're in an environment that can tolerate it, using salvaged heat sinks and salvaged fans inside a salvaged power supply enclosure would likely work well.