Here are some ticket agencies - some only have tickets for USA some just Europe and UK
If you are about to travel overseas you can check whats on and book your seats before you leave

FROM THE BOX OFFICE
have a bespoke system which connects directly with the venue’s Box Office and completes the bookings live with the venue. This technology allows our customers to book up to the LAST MINUTE before the performance, that is, literally up to 5 minutes before the performance starts – and customers can view their seat numbers before they buy!

TICKET LIQUIDATOR
sell tickets to the world's biggest concerts, sports, theater events and more. We're an industry leader with a great selection.