Tess,

If the mill knows you are ordering for a kilt then yes, most I order from will ship it as a single width piece. But it is best not to assume that and be sure to ask for it.

You should be able to get your Ulster tartan double width, unless you are ordering from House of Edgar, who does it 30" wide, but the per yard cost works out about the same as it did when they were weaving double width.

(Ulster is symmetrical, btw).