Rule number one of buying seconds-hand tickets: avoid buying from touts. They operate illegally and will charge you extortionate prices for what might not even be a real ticket.
There are plenty of legitimate websites that now offer you the chance to purchase your ticket as-new, second hand over the internet.
A number of sites let people sell their tickets second-hand cheaply, so if you're lucky you could even pick up the tickets you were after for less than the face value.
Any of these sites that specialise in second hand tickets for a range of events are worth a try:
Check each site to find the best price, and don't forget to factor in booking fees. Each site may charge different fees, so ensure you have checked the full amount before you make your purchase.
Another option that's always worth a try is sites like Gumtree and eBay. The price you pay here will depend on how much the seller is willing to let them go for, or how lucky you get through an auction.
Second-hand isn't the only way to get tickets to popular shows for less. Many sites will offer deals and offers, and it's always worth keeping your ear to the ground and comparing prices to find the best deal.