It's so sweet of you.Here is the suggestion ,there are a lot of kitty sunglasses on Ebay and Amazon,you can visit them then insert "kitty glasses"in the blank,then there will be amount of glasses listed,then you can choose whatever you like.
But if you don't like buying goods online,you can go to hello kitty real store, that's a paradise of kitty stuff which are all high-quality thing.Besides ,you can take your daughter to Disneyland,you can enjoy a nice day there then buy a kitty glasses there as a souvenir.There has to be plenty of kitty stuff there,I have seen them once.
What's more, if you want to make that special, you can ask glasses store for help,just ask them to make a unique one for your daughter.They would ask the supplier for help if they can't do that them own.