Go to http://rswww.com >Mechanical Products & Tools>Workshop Consumables>Engineering Materials. I would link it but the site is built on Broadvision which does not allow linking due to session ID's which last 20 minutes. RS is expensive but they do have most parts, otherwise look to your local Iron Monger.
My two local B&Qs have a really naff selection of sheet metal, their thickest alu sheet is only 0.5mm... RS is good if you are going to get a lot of Alu in one go. and aluminiumsupplies.co.uk is expensive for small quantities as you get stung on deliveries. Site I would recommend is chronos.ltd.uk, they have a good selection of 300mm square sheet metal, and delivery is included. They have lots of other useful stuff as well, such as tap and die sets, and the milling tables that dutchCedar uses for BadAssumption. PS. My first post here, although I've been lurking for a while... I'm planning my first case it will be a modular acrylic/alu effort, expect a worklog soon...
