About the only time you will find tartan fabric at a local store is when Joann's stocks "schoolgirl skirt" material in the fall. I'm going to PM you about some of that I have on hand that might suit.

Other than that, eBay can be a good source but it's hit and miss, and you need to question sellers. One fellow loves to list tartan as "kilt weight" but in response to my question said it was 10 oz fabric. Real tartans are not inexpensive but you can sometimes find "fashion tartans" (typically just called plaid) at a reasonable price.