I got 5 yards of 60-inch wool tartan off of ebay for about $35.

Search ebay for "wool tartan", make sure that the stuff is 60 inches wide, and get 3-4 yards of something that appeals to you. Don't rush it, new stuff comes up all the time. You can also get a wool blend, and that might cost a little bit less.

Alternatively visit Fraser and Kirkbright and buy some of their remnants, or the Scottish Weaver (F&K's USA distributor) and get some of their remnants.