Pictures of the Birka posaments can be found at the Swedish History Museum website (http://historiska.se), along with a link to several resources, including Birka III (the textile finds).
Posament group: https://www.facebook.com/groups/679673238710312/
Ethni has a lovely spreadsheet of posament finds, relevant links, and she has started adding tutorials. http://eithni.com/posaments/
TenntrĂ¥d comes in various dimensions, with .30 mm being fairly common. The percentage of silver versus pewter also varies. I recommend the following links for purchasing it:
Range of sizes and metals http://www.pewterofsweden.com/en/