I've played AO quite a lot over the past year but have come across a platinum ingot only once. I got it in a mission, but I forget if it was from a chest or from a mob. I've never found anything that produced a platinum ingot when melted. These ingots appear to be extremely rare. If it was my engineer or trader who had found it instead of my enforcer, I would have made a ring out of it by now. Maybe this weekend I'll get someone to transfer it to my trader.