https://www.nps.gov/valr/planyourvisit/fees.htm This is the only site you need to “buy” Arizona tickets. You can order them 60 days out for a small online booking fee. If you miss the first lot, then they release more tickets 24hrs out from the date - miss out on these and you have to line up on the day.
Missouri tickets can be bought the day you go. Are you aware that you are currently unable to disembark onto the Memorial at present? The launch currently goes around the Memorial whilst they repair it.