The problem with purchasing fabrics in N. America is that there are so few domestic weaving mills left that the vast majority of fabric are shipped in in great bulk. Entire shipping containers of one type of fabric in color at a time.

The only places that can handle that sort of volume are companies called "Jobbers". They warehouse, cut and ship fabric to manufacturing companies. These Jobbers seldom sell fabric at retail to individuals.

Then, in many cases what ever is left over at the jobbers is sold to retail fabric stores. In some cases when a shipment arrives at your local fabric store the sales staff have no idea what will be in the truck. It as bought by the head office and shipped to them. JoAnns and Fabricland are two examples.

My source for much of the Poly/Cotton and Bull Denim Cotton fabrics I buy is a Jobber. I am a business with a Tax No. and can buy wholesale. And I buy in larger quantity than someone who will only make one Kilt from one color and type of fabric. So me giving you my sources would not do you any good.

The best advice I can give is know exactly what composition and weight of fabric you want for your Kilt. Know what it feels and looks like.
Then hit your local fabric store on a regular basis. Ask the sales people when they expect their next shipment and be there the next day. Go through the store and touch the fabrics till you find exactly what you are looking for. Don't ask the clerks for 13oz, 65/35% Poly/Cotton 2x2 left hand Twill. They won't know what you are talking about. They will only know what is printed on the shipping manifest and what price to put on it.

For on-line sources you usually get even less information that walking around your fabric store because you only have the most basic information and sometimes a photo of the actual fabric. (and sometimes the photo is not from the exact fabric in the description but one of the same color left over from the last time the website was updated.
So, ask questions. Make phone calls. If you don't get a good enough description of the fabric you are looking at buying then pass on by and go to the next listing.

It took me two years to find my source and I'm sorry but with the way the fabric industry is in the world today that is not uncommon.