I think Joann's Fabrics carries them here in the US. Not sure where you are at.

When I embroidered on cards, I used plain cardstock, cut it to the proper size using a paper cutter and folded them carefully. The office supply store has envelopes, so get them first so that you know what size to make your card.
I loved making my Christmas cards last year and plan to make more this year. Good luck! ~ Nancy