So I recently completed my first attempt at a Viking tunic. Unfortunately, despite following the pattern, it is at least a size too small. I'll get some photos of the tunic up later.
But if you're wondering how I came up with a pattern in the first place the answer is...
Lots of different places.
Here's a blog from a man who has opened a business devoted to reproducing Viking fashion for the Renn Fest, SCA, and Re-Enactors: http://vikingtunics.wordpress.com/ He's very informative about what it takes to get started.
There are any number of links out there for you to get instructions. The UK Re-Enactment group, The Vikings has a ton of resources available on its website. Even the North American branch re-directs you to the UK website. http://www.vikingsonline.org.uk/resources/authenticity/basickit/index.htm
Another UK group with excellent information is Regia Anglorum: http://www.regia.org/main.htm
You can find many more links through this directory of Viking Websites: http://www.dilettante.info/vikinglinks.htm
There are, of course, links to sites that will sell you completed clothing. To each his own. Personally I am very interested in the artistic aspects of re-creating a Viking costume. I would prefer to make as much as I can. In theory, I could make everything. This is what many people did 1000 years ago: make every article of clothing and ornament. However, I recognize that my ability to sew is already ahead of my ability to do leatherwork or cast jewelry. So I have bought, and will buy, various bits as necessary. But eventually I should be able to do it all and make it look pretty. At least, that's the plan.
However, the ultimate answer for which pattern I chose to use for my first attempt at a tunic is this:
http://www.amazon.com/McCalls-Patterns-Childrens-Costumes-SML-MED-LRG-XLG/dp/B004OA79A6/ref=pd_sim_sbs_ac_15 I used the pattern for the lower right corner tunic. Although I did make some initial adjustments to the sleeves so they would taper toward the wrist rather than billow, and I left out the belt. When I get the size right, it should make an excellent undertunic of linen. It isn't right for the wool outer tunic, although a correct undertunic pattern should give me something to judge the proper size for the outer tunic.
I'm also using the trouser pattern, so we'll see how that turns out.