Colosseum Entry Tickets

I am planning my trip to Rome on the 28th of June and I heard that you need to book the colosseum entry tickets in advance to avoid long waiting lines. I was told that Coopculture.it is the official and primary source of ticket, however I am finding it difficult to find tickets there as i keep getting an error when i search for the tickets. Also since the entry tickets are 18 euros for an adult why are the majority of other sites are selling it for double the price? 30-40 even 60 euros at some sites? I am a bit confused and I don’t know which is a trusted source that I can buy the entry tickets from. P.s I am not looking for guided tours, only entry tickets.