Thank You Cards
Order thank you cards when you order your other printed items. It's usually cheaper this way. You can buy thank you cards to match your other invitations or you can buy cards that match your theme.
It's a good idea to get thank you cards that don't necessarily indicate your XV años. If your thank you cards have a picture of a princess in a quince gown, then you can't really use the left-overs very easily. A more generic card, even if it matches your theme can be reused at any time.
You can make it easier to write your thank you notes by keeping good records of gifts and contributions to your celebration with a gift log. (I've included one in the bonus planning pages that come with Planning the Perfect Quinceanera.) Write thank you notes as soon after the event as you can and make sure to put a little time and effort into making the notes sound very personal. Even though you may have to write hundreds of them, you should still do your best to make each card sound unique.