I buy card jackets from
www.clearbags.com . Great prices and quick shipping. They have just about anything you could think of.
I always decorate envelopes to match my cards. I put the card in, then slip the envelope in behind the back of the card with the decorated front of the envelope showing.
Card jackets are great, too, because the buyer can look inside the card before they buy. The jackets are slick, too, and slide in and out of the racks more easily than bare cardstock.
By the way, when my buddies and I do craft fairs, we remove the card jackets at the time of sale and no one complains. We tell them the jackets are there just to protect their beautiful cards until they are "in the bag".