If you happen to be passing through Ulaanbaatar (or live there and work there) you could do worse than spend an hour in the Postage Stamps Museum. It's on the first floor of the main post office on Peace Avenue (not signposted, take the stairs on the right as you walk towards the main counter). They have a great display of Mongolian stamps and postal history dating from before Mongolian sovereignty to the modern day. You can buy many of the stamps at the desk on the ground floor, and the staff are helpful and speak English. They didn't allow me to take photos inside, sadly. You can also visit the site at
www.mongolstamps.com, where you can buy on-line.