When I make cards and a money envelope is required I simply go to my local bookstore, and buy smaller envelopes that you receive in the mail, about a card-size...
Then I measure it to my card before cutting it and folding, just to make sure the envelope fits. I use double sticky tape and glue it on the card, then if I want, I just decorate it with embellishes, it´s a lot cheaper than buying some envelopes especially made for card making.

There you go! I hope you understood this:)