You probably shouldn't do any saltwater puffer tank unless you are willing to mess around with salt and a refugium ($$$$).
Stick with light brackish or freshwater puffers. What kind of puffers are offered in your location? Is 3inches big enough for you? All of the active brackish and freshwater puffers are around 3 inches. If you want something bigger for your 40g, then it will end up being an inactive/lurker puffer. Are you trying to get a puffer with a good chance of it accepting tankmates (because almost all puffers are territorial and better off being in a tank by themselves)?