Staples do quality card that will suit your purpose fine. They have plain White, pastel, or full colour to choose from, also partly printed ready for you to add a personal touch.
I use a Their "Rey-adagio" brand, 160gm for the outer card and 80gm for the inner leaf. For the really special cards, then use "Photo" card with a satin, or pearl finish.
Do them in A5 size, (1) sheet of A4 folded in half for the outer card and then the same for the inner leaf, but trim down an inch on all four sides. Use a little paper glue to attach the inner leaf or Use a hole punch and thread with a ribbon or coloured cord. Regards. j.