Songkick lists where you can buy tickets:
I prefer to buy them in person from the venue or Stargreen box office in central London.
If they are not in Stargreen, I like Wegottickets, SeeTickets or TicketWeb.
I hate using Ticketmaster.You have to wait, then is says they no longer have tickets and would I like to buy them from a touting web-site.
If I manage to get them they then offer discounts from a company called Complete Shopper. These are just scammers.