Check local gift shops, drug stores and convenience stores for postcards. These places often have a variety of post cards for sale that might be a perfect fit for your needs. Don't forget to check the gift card section. There may be packages of bulk postcards for sale, saving you time and money.
Check online retailers like Zazzle, CafePress or Flickr. These websites allow you to customize your own personal postcards based on photographs uploaded by you or others. If you're taking a vacation to the Florida Keys, for example, you might find the perfect beach sunset to capture the mood of your trip that you can turn into a post card. If you choose to order your postcards online, plan ahead to allow extra time for delivery.
Purchase a book of stamps from your local post office branch. You don't want to have to scout around the hotel for stamps once you get there, so it's best to do it in advance. Speak with the post office clerk and tell them where you're going and the type of postcard you're sending; they'll make sure you have the proper postage. To make the process even easier, you may even want to adhere the stamps to the postcards ahead of time.
Address all postcards to their intended recipients. If your postcards are stamped and addressed before you even get on the plane, you won't have to worry about any of the details when it comes time to write them. All you have to do is write a short note and drop it in the mailbox.
Bring along a few extra postcards just in case. You might remember someone you hadn't thought of, or your travel companion may want to send one. Whatever the case, it's better to be safe than sorry.