Easiest to find will probably be a board backed A4 envelope. I usually stick a sheet of cheap card in the envelope as well, on the other side of the print to the board back. I'll generally stich a sheet of plain paper in on the print side too - which I try to face the board back.
Pretty much any stationer will carry the envelopes, either singly or in packs.
If it's a one off, PM me your postal address and I'll post an empty A4 board back up to you with the flap taped down so you can use the sealing strip. Any postage can be put in a deserving charity box!