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 from Northshield has a lovely spreadsheet of posament finds, relevant links, and she has started adding tutorials. http://eithni.com/posaments/
Tenntråd comes in various dimensions, with .30 mm being fairly common. The percentage of silver versus pewter also varies. I recommend the following links to purchasing:
Pewter and other metals http://www.pewterofsweden.com/en/
SCA member in AnTir who also carries my favorite Epic Wool
Ethni’s etsy shop https://www.etsy.com/shop/TheExcellentFrog?ref=hdr_shop_menu