Question in title. I've seen some people in a discord I'm in say that you need rare mana prisms but from looking at the wiki it doesn't seem that that's the case. Just looking for some clarification, thanks in advance.
No, you don't need rare prisms for sure.
I think that you might have misunderstood those people in Discord - apparently if you reacquire (on a rerun, for instance) certain costumes you GET a rare prism.
https://www.reddit.com/r/grandorder/comments/8xczfo/rare_prisms_without_having_to_burn_units_are/
Purchase the permit item that lets you craft it from the event shop for the 7 counterfeit heroic spirits lite rerun, and then it costs 10 Magical Cerebrospinal Fluid and Stingers of Certain Death, which aren't in the game yet, and 5 Hearts and Grease.
You can find info about all the costume dress here:
http://fategrandorder.wikia.com/wiki/Costume_Dress
