There is a shop down the Walworth Rd. (not SE1, I know) dedicated to such things. Can't remember what it's called, sorry. It's quite a way down on the right, past M&S I think on the opposite side. I have used it, it's pretty good.
There's a place on East Street where I've found decorations and balloons. The 99p shop on Walworth Road near Baldwins (can't remember the name maybe Sammy's?) often has a selection and also has a gas cannister for the balloons. Also the 99p shop in the Elephant & Castle shopping centre often has a range of decorations. Or if you have time I can recommend a website: www.partydelights.co.uk who I found very good value and quick delivery.