Tickets have been going for £20-£30 on Gumtree in the last couple of days. There's clearly a surplus of supply. You should be able to get one at face value or less tonight.
I wouldn't say there was anything hugely unethical about buying off a tout in the street either. They take spare tickets off people whose friends couldn't make it etc, and sell them on for a mark-up - probably around £10 tonight if they're lucky.
The worst touts are those who snap up tickets and sell them on E-bay. Never buy one off them!