Do you know about vistaprint.com? They're an online printing system, and one of the things you can get from them is postcards.
Usually, the minimum order is 100 for $25 or 250 for $50, which isn't a very good price. But sometimes they have a public sale (one is going on now) and you can get 100 for nearly free (shipping only), and that includes color front and black reverse printing. This direct link may or may not work: http://www.vistaprint.com/vp/ns/splash/splash_postcards.aspx?xnav=menu
If it doesn't, start at vistaprint.com and navigate to Business, then Postcards.
Sometimes if there isn't a public sale going on, you can enter by "www.vistaprint.com/vip
" and get a good offer. Or substitute the name of the month for "vip" in the above.
Another approach is to order anything from them first ... they'll start sending you promotional email if you permit it, and there will be free or nearly free offers for postcards about once a month. I suggest you get some of their free business cards ... if you let them print a tiny ad on the back, you can get some nice cards for just shipping, after which the promotions will arrive regularly.
Also, they seem to permit you to place more than one order for 100 free cards. I know I've ordered two sets the same day.
Now when ordering postcards for use as QSLs, notice that the width is correct but the cards are 4.2 inches high. Pick a design and place the text so that you can cut the bottom 0.7 inch off. I do this with a paper trimmer a few at a time, but you could also take your 100 cards to a local printer and ask them to cut them for a few bucks.