www.download.com
Or you could search google for greeting card software download, alot of companies now offer their software via a secure download.
If that doesn't work, you can try this.. You really only need to print on the front of the card and inside. These parts happen to be on the right side (in landscape view) of the page. Just change the margins to 4.25" (half of 8.5") from the left and center the art and text. When you fold the paper, the front of the card and the greeting inside are formatted just like a store bought card. For the future, you can pick up cheap greeting card kits and/or paper on eBay. Keep in mind cardstock is heavier than ordinary printer paper, and you may need to change this setting in your printer window (it pops up and you hit "ok" to print). A tip for images: Google image search is fantastic for this. You can find their favorite food, music artist, city, sports team, whatever.
Always(!!) do a test page on extra paper so you know which way to turn the paper after the first print (because to fold it right, you'll be printing on both sides of the page). Good luck!