The beer and wine selection is pretty much the same at the two big supermarkets in Stornoway, Tesco and The Cooperative.
However, my recommendation is buy your beer at Tesco, as they have a much larger selection of Scottish Ales,and they're cheaper. For wine, the opposite apllies, a much better selection is at the Cooperative.
Harris is of course well known for its "harris Tweed" The local tourist office can give you the addresses of local weavers.
What to buy: harris tweed - some of the local weavers still use the indigenous plants to make the dyes they use. It is not cheap but you get the real thing!
What to pay: sorry I cannot give prices as it is a while ago I was there