For serious stocking up (including alcoholic beverages) you need to go to Oban, Fort William, Bowmore, Mallaig or Tobermory with their reasonably wide range of shops and supermarkets. But don't forget the village shops which need your business.
More specialist shops worth a look are at:
Ardalanish, South and East Mull: isle of Mull Weavers (or walk from Bunessan, West Mull)
Tobermory, Sound of Mull: Chocolate factory, Browns for a brilliant window display and loads of ironmongery stuff along with the whisky, and the Mull Bakery and Deli for your morning croissant.
There are a few bookshops left standing, and rather good ones too. It is well worth browsing around in:
Bowmore, Islay: Celtic House
Iona, West Mull: the book shop across from the Abbey
Fort William, Upper Loch Linnhe: The Highland Bookshop
Scalasaig, Colonsay: House of Lochar
Tobermory, Sound of Mull: Tackle and Books — my favourite
And to ponder on literary inspiration visit Barnhill on Jura to remember George Orwell and 1984.
There are also a several galleries and craft shops around the place:
Calgary Bay, West Mull: Calgary farmhouse forest walk
Crinan, Firth of Lorne: Crinan hotel has art exhibitions
Eigg, North of Ardnamurchan: craft shop on the pier
Muck, North of Ardnamurchan: craft shop
Poll an Dùnain, Loch Leven: F.W.Holroyd Art Gallery, and Starfish Celtic Harps
Salen, Loch Sunart: Resipole Studios
Strontian, Loch Sunart: Woodland Pottery
Tobermory, Sound of Mull: An Tobar and Tobermory pottery
These are of course all rather tempting, so avoid taking your credit card when visiting them (I must confess to occasional and not inexpensive lapses).