Hey there. You can buy them online (1 for each public day) but the ticket booths at Makuhari are literally right in front of the line where everyone waits to get in, online ticket or not. So I usually just get mine at the convention and hop in line. Up to you though.
Also, if you didn't know, cosplayers are 'supposed to' change into their costumes at the convention. There's change rooms where you have to pay to get a badge that says you're a cosplayer.
Last year cosplayers had the option to get their ticket, wait in line and then change inside the convention center OR use the change rooms before the line, wait in line in cosplay and then be free to roam around to your heart's content.
Upcoming events: Wonder Festival, Katsucon
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