I am definitely not precious about the materials I use to make my work, but every so often I find myself in need of something more specialised. L.Cornelissen & Son is a beautiful and well stocked art supply shop with extremely knowledgable staff.
One of my favourite places to snoop around is the food market open on a Thursday, Friday and Saturday. It's always bustling with crowds so great for people watching. You can get the best of anything here which is why it's hard to resist spending a small fortune.
The best thing about any charity shop is that you never know what you are going to find. I have a well tested route around London that includes all of my favourite charity shops. This Oxfam has a great selection of books and often turns up some interesting clothes too.