Like many Americans, I have a rather mixed bag lineage. My father-in-law would call me a "dog's breakfast", (an endearing term, I assure you) meaning I've got a little bit of everything. My dad has always been interested in his Scottish geneology, and gave all three of his daughters Scottish first names. While I was sort of unwilling to plan my wedding colors around a tartan plaid, it would have been nice if in some small way, I could have worked my Scottish lineage into my wedding. If I had known about the thistle pin at the time, I would have definately incorporated into my outfit.
The story goes that a band of invading Vikings were sneaking up on a group of Scottish soldiers one night back in like the year 9, and one of the Vikings stepped on a thistle. He yelped in pain, thus allowing the Scots to defend themselves and get away unharmed. Since that time, the thistle has been a symbol of Scotland, and the thistle pin has been a badge of pride worn by Scottish women (and sometimes men).
While it isn't particularly a wedding tradition, it seems like a subtle way to honor a Scottish heritage, stopping just short of putting your fiance in a kilt.
Many, many, many of these items can be found on ebay under the search phrase "thistle pin".