The Principality of Sealand is a small nation located on a derelict sea fort of the coast of Great Britain. Maybe. It depends on who you ask. If you ask the folks on Sealand they will tell you that they are a sovereign nation, and that England has even implicitly recognized their sovereignty in the past. If you ask every other nation on Earth they'll tell you that those guys are crazy. Sealand is the most well known micronation, a small entity that claims to be a nation but that is generally not recognized as such. Sealand arguably has a better claim to sovereignty than most micronations but I doubt they'll be getting a seat in the UN any time soon.
Anyway that link up there is their official site. It's got info on Sealand's history and buy stuff. They sell coins, stamps, and even titles of nobility. The perfect birthday present for the royalty in your life. Check it out.