Hamish, I've only started looking into this, but a couple of hours of poking around the web and using the Scottish Tartan Authorities' tartan search engine reveals an astonishing number of absolutely wonderful non-clan-associated tartans. Who knew? I certainly didn't! It seems to me that most anyone can find a few tartans that they'll like that are not linked to a Scottish clan and have real connections with something in their life.

For example, any USA citizen, or resident of the USA should be able to wear the United States tartan, and it really is a beautiful pattern. I'd bet anything that the cloth is reasonably available, which might not be the case for some of the other, non-clan, non-Scottish tartans.