As a witch, I find choosing Yule cards quite difficult. Many say "Happy Christmas", while I would rather wish people "Merry Yule" or "Joyful Solstice".
With the lack on anything better, I generally settle for sending "Season's Greetings", especially if the design on the front of the card is appropriately pagan. The picture on the left is called Winter Star and is on a pack of 10 cards sold in aid of Epilepsy Action, for £2.95.
I like to buy cards in aid of charity if possible. Cards for Good Causes sells charity Christmas cards on behalf of more than 320 UK charities and is the country's largest multi-charity card organisation. It gives back at least 79p in the £1 from the cards sold to charities. CFGC has shops all over the UK and you can also order cards online at http://www.goodcauses.co.uk/
If you want to buy pagan greetings cards, you can actually buy Yule cards at Amazon.
E-cards are, of course, more environmentally friendly and cheaper to send that putting bits of dead tree in the post. They are also great if you find you've left things too late and missed the post. There are some gorgeous pagan ones around. Here are a couple of websites with e-cards suitable to send for Yule or any other occasion:
After the festive season is over, you can always reuse beautiful cards you have been sent, by turning them into your own personalised divination deck. For more details on how to do that, see The Bad Witch's Guide to Fortune Telling