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If you make your own cards be sure the size is 3.5 x 5 inch

To find places that make QSL cards, just Google it. I (KG7KA) make my own as do many of us so I can't recommend any specific service. 

 Notes from KG7KA: I make my own cards using CorelDraw and a laser printer using 80 lb card stock.
I make 4 up on a 8-1/2 x 11 stock and cut them up to standard size of 3.5 x 5 inch. 

 Wikipedia has a good discussion of QSL if you are not familiar with it. 

 This site explains QSL cards pretty well 

Discussions of QSL protocol Here and Here

 Here is a How To Guide 

 This is the Ethics and operating procedures for the amateur radio operator
Its a download PDF file of 68 pages. QSL use is discussed all through the document as it relates to proper contact exchange information. 

 There are lots of services available for making cards and providing various services like this one that send cards for you.