Honestly, for the majority of tartans that aren't listed with Bear or USAK, it's pretty much a shot in the dark as to finding them. Everything else is wool.

If you print-out the picture of the Carnegie tartan and take it with you to every fabric place that you go to, you might just get lucky and find it... or come REAL close. You might even try eBay under "plaid fabric" or "plaid material" and enter the base color to weed-out the others.

That comes from a guy that used to have to search everywhere for tartans for the business!