Ball costume

[mornings]

So I'm planning to cross-dress cosplay as Draco Malfoy for the Victory Ball. I don't have a suit (let alone robes) or a tie, so I'm going to need to buy them in London. Where I could find a very cheap black suit and a green tie?

Of course, if one of you happens to have a little brother or a kid with a spare suit... I'm quite short (162cm) and normal weight. ;)