I have purchased stamps from the United Nations (NY) in the past, and, while the service often takes 4-6 weeks, it is alway worth the wait as they typically send mail out with a wonderful selection of stamps, nice placement, and great postmarks.
A set of stamps was released to commemorate the 70th anniversary of the UN in 2015. Two of these large square stamps were on this envelope, finished off with a special 70 postmark.
For blogs with more commemorative stamps, visit Sunday Stamps.
Bonus commemorative stamp on yesterday's blog post.