Hi Emily,
Congratulations on your very first Magical visit to Walt Disney World! Be sure to stop by your Disney Resort hotel's front desk or Guest Relations in any of the theme parks to pick up your complimentary "1st Visit" celebration button. Be prepared for all of the well wishes you will receive while you are having the time of your life!
I love that you are going to buy postcards while you are there! I did the same thing on my first visit, which was during my honeymoon, and my family back at home got such a kick out of receiving them. You are sure to find some really great ones!
Emily, merchandise can vary from time to time but in general I have usually seen postcards for purchase at the below locations:
I am sure that there are many other places that you will encounter adorable Disney themed postcards while you are visiting. In general, the shops and kiosks near the entrances of the theme parks usually have postcards for purchase. Additionally, postcards are often for sale in most Disney Resort hotel gift shops. One great resource is to ask a nearby Cast Member for help and they will be super helpful in helping you locate some nearby postcards.
Have a Magical vacation, Emily! If you have any other questions, please come back and visit us here again at planDisney. We are always excited to help you plan out your next adventure.
See Ya Real Soon,
Brenda