For serious stocking up (including alcoholic beverages) you need to go to Oban, Fort William, Bowmore or Tobermory with their wide range of shops and supermarkets. More specialist shops worth a look are:
Ardalanish, South Mull: Ardalanish weavers (also walk from Bunessan, West Mull)
Tobermory: 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, and rather good ones too. It is well worth browsing around:
Bowmore, Islay: Celtic House
Scalasaig, Colonsay: House of Lochar
Tobermory: Tackle and Books - my favourite
And to ponder on literary inspiration visit Barnhill on Jura to remember George Orwell and 1984.
There are a several galleries and craft shops around the place:
Ardminish Bay, Gigha: Craft Workshop and Gallery
Calgary Bay, West Mull: Calgary farmhouse forest walk
Crinan, Firth of Lorne: Crinan hotel
Eigg: craft shop on the pier
Fiunary Rocks, Sound of Mull: Morvern Gallery
Fort William, Upper Loch Linnhe: Lime Tree Studio
Muck, south anchorage: craft shop
Port an Dunain, Loch Leven: F.W.Holroyd Art Gallery, Starfish Celtic Harps
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).