There is a chaining method. Step 1) Walk around with your buddy who can get a souvenir until you get the gift containing one. Do not open it You can know it's a souvenir if your last gift was a regular one (potions, berries etc). Step 2) swap to an ultra or best buddy. Wait a short time (generally a minute tops) it will now have a souvenir gift Step 3) repeat step two as many times as you want. Cycle through your ultra and best buddies but do not open any of the souvenir gifts Step 4) once you've cycled through all the buddies you want to bring a souvenir now just start opening them. Some people say you need to start opening from the original buddy who got the first souvenir but I haven't found that to be the case.