Anyone help me with a card blank sourcing question? A firend is getting married and making her own invites. She's seen a card idea she's quite taken by, but it's cut from A3 rather than A4 cs.

I'll try and describe it! It's a square format when closed, but opens up into three, with the bottom section forming a wallet-style enclosure (so you can pop in address details, directions, gift list info etc).

Any ideas?