Just ditto what others have said. If you purchase from a reputable tartan dealer, they'll know what you mean when you ask for the Muir or MacWhirter tartan. You won't need to supply a threadcount or anything, just let them know what weight you need. Muir is available from stock from more than one mill in a variety of weights. MacWhirter will require weaving, but for a full bolt that won't be a problem.

Most places (STM included) only price per yard on their web sites. If you phone in and talk to someone, you should be able to get a discount on a whole bolt of cloth.
http://giftshop.scottishtartans.org/yardage.html