Hallo Philbo-
Highland history is more than confusing, and tartan and clan structures make it more so. One point you might want to consider: clan membership was not only by blood, but also fealty, much like serfs to the lord in the castle. In the Highlands, there were few castles, and the Big Men were called 'lairds,' but the system was very similar.

To the point, if you can find a clan chief you are willing to pledge to, you would wear his tartan to advertise the relationship. Or, if there is any Scottish blood in your family tree, a few drops will grant you membership.

Genealogy on the web is surprisingly easy and almost as addictive as kilts. Luckily, you can do both. Your kids can get into it anyway- the stories they learn today should be heard in tomorrow's tomorrow.

I've looked into designing an official tartan. 'Expensive' just doesn't even begin to describe it. The GB£55 quoted from Locharron converts to US$100 x 11 meters. And that's just for production.
Registration is another US$300, at least through http://www.tartandesigner.co.uk/registration.htm and only GB£27 ($54) a meter after that. They offer design services for GB£350.

Thumbnail estimate for the cost of a finished dress kilt in your custom tartan: $1700.

Finding and joining a clan or organization with an established tartan might be a wee bit easier... or maybe a digital ready-weave loom for low cost short run tartan ;-)

-G