Here is by far the best site for Metallica Tickets. Why? Because not only you probably already have an account with them but also because they are safe (you can see ratings and testimonials) and have the biggest selection of Metallica tickets ...

Stubhub isn't bad either. They are safe and have a good selection of tickets.

Craigslist is another popular option but, with all the ticket scams going on, I would be very careful and would recommend instead using one of the two previous options.

Hope this helps,

Ed

PS: If you'd rather see Metallica for Free and even meet them, find out about getting Metallica Backstage Passes