Keep your eye out at the various dollar stores. I pick up a lot of Disney character things there (purses, pads, nail art, pens, etc.) I have our next 3 trips planned over 2 years. I have a box for each trip and buy accordingly, We are different groupings each time so I buy different amounts and pay attention as others said to sexes and ages.
I started a tradition that if I am not careful will be costly. Each night when we get back to the room, the kids each find a note addressed specifically to them with something about their day and it is signed by a character and has a gift. for example, the note said "hope you saw me in the parade today down main street. Love Cinderella" it had a cinderella backpack with it.
I usually play stupid and run back in the room with an oops I forgot something and make everyone wait outside while I "help the character" put out the gifts. It really adds magic to the trips and if I do it wisely costs me about $20 for 5 days (approx. 10 items) and than cuts down on souveniers to almost nothing.
Good luck
Sandy