I can't give a definitive answer, but Chippenham booking office once sold me an off peak day return from Paisley Gilmour Street to Irvine. So Id say if it is available at ticket offices in Yorkshire then it should be avaiabke where you are
I purchased a Spirit of Scotland rover from Newton Abbot Station, so you should be able to purchase just about any valid
UK▸ rail ticket from most ticket offices