Bridge: Worldwide Music Connection — online database of opportunities includes 900+ competitions.  Available by subscription, check out with free trial

Musical America Worldwide — International Directory of the Performing Arts; subscription database

World Federation of International Competitions

Some specific competitions that offer pre-professional management or managed touring:

Astral Artistic Services

Concert Artists Guild

Honens International Piano Competition

The Pro Musicis International Award

Young Concert Artists

Van Cliburn Competition for pianists