Mint Royale was a prize from the Cash Shop RIG "Digital Rainbow". That was back in June 2012, so you're not likely to find one for a good price...but there's hope. Gaia Staff recently took a ton of those old prizes from Cash Shop RIGs that are used in Alchemy, and threw them into the Alchemy Fusion Cache, which can be created through GoFusion from 15 Heart Charms and 15 Star Charms. I haven't actually heard of anyone finding Mint Royale in them, but it still could be in there.