I've had good luck over the years with my Harbor Freight conversion pot. It took minor modifications to get it going and has served me well for over 5 years. If you go this route, be sure to replace the safety valve and put a good pressure regulator on it. I also marked each lock down and put the same lockdown in the same spot on the lid each time I use it. Some suggest a little Vaseline on the gasket, but I have not bothered. Keep the pressure under the published limit of the tank - I rarely push it beyond 40 lbs which is more than adequate for most resins. I also suggest making a HDPE or plywood bottom for the tank to provide a flat surface for holding your molds - and add a small bubble level into the bottom piece in the middle to keep the tank level so resin doesn't run out of a full mold.
I also have a California Air Tools 5 gal tank, and if you feel you will be doing a lot of casting, I highly suggest this tank especially if you will be doing vertical 8" bespoke pen blanks - they need the extra height.