IoS is a copyrighted tartan so legal production will be limited by contracts with the mill. That means you're not likely to find it at any fabric stores. You may find some being sold on eBay - I have had good luck purchasing other kilt fabrics produced by House of Edgar there. Your other option will be contacting one of the many kilt shops and kilt makers that routinely order material from the mills. I suspect many of the kilt makers and kilt shops that are XMarkers can order fabric for you.

I suspect someone who knows more will chime in with even better information for you.

Cheers!

P