When I first started trying to sell my items wholesale, I put together a catalog and saved it as a PDF and uploaded it to my ipad so that I had the entire collection there. Since I don’t just sell cards, but coasters, notebooks, bookmarks and such. Then I went to the container store and found a nice box that was about 9x6x2 and I would bring along 1 or 2 of each type of product that way they could see the quality and if they were interested or had time we could sit down and look through everything or I could email it to them.
If currently you have about 18 cards another option would be to get a large portfolio maybe 11×17 and attache all your cards onto that so that you can easily open it up and they can take a look at the body of your work in a few glances.
Store owners are always in a hurry, but if your designs catch there eye they will spend plenty of time with you.