Gutermann is available in almost every fabric store chain in the I have every heard of. I had no trouble finding it in the UK during my visits. Some small, or local sewing shops don't carry it but the larger ones do.

Coats & Clark double-duty thread is new within the last 5 years but is also carried by the larger shops.

What you want for kilts is the highest quality thread you can find. 100% Polyester preferably. DO NOT use cotton wrapped thread of any sort or brand.

Try Fabric Land
11 Bartholomew St.
Town Center
Newbury