Legitimate event ticket sites?

I’ve never bought tickets online before except to the local cinema. I want to get some tickets to the Ringling Brothers Circus when it comes to town in August.*

Are there any sites you’d recommend or warn against? Am I ok buying from, say, TicketZoom? Why are they so much cheaper? Is there any question of authenticity? How come they have tickets and TicketMaster doesn’t?

Any advice and recommendations appreciated. I want to order right away.

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* I can be highly suggestible. I’m reading Water for Elephants, by Sara Gruen, and it made me want to go.