I’ve used Walgreens for ordering photo cards on numerous occasions. This would be a great option for personalized thank you cards. They have options to create cards that you could write a personalized note in, or simple photo cards that would have a general message that would be the same for everyone.
Before you check them out, be sure to Google for photo coupon codes. If you are on their email list for photo deals, this is a great way to get weekly coupon codes. Below is a coupon code that is good until November 30th.