The type of wedding thank you cards you will be sending to your guests will actually determine when you may want to purchase them.
Note cards or thank you cards can be purchased at the same time you order your wedding invitations. Your invitations may even have them available in a matching design. Note cards will usually have your first names on the front with the inside to be used for either a pre-printed note or your own handwritten note.
If you choose to send photo thank you cards, you will have to wait until after the wedding so you can have them printed with a favorite wedding photo to share with guests along with your personal note.