You can make your own invitations. You could do initials, baby blocks or rattles. Another idea is make one in the shape of a baby onesie. You can decorate them to look like a boys or a girls and it's really fun and easy. They have free templates online that you can use.
You could talk to her mother and get some cute photos of her when she was a baby, and put that picture along with some ribbon or baby-like stickers around it. That makes the baby shower both about her and the baby and your friends will get quite a laugh out of it.
I absolutely love baby shower invitations that include an ultrasound picture of the future baby. Also make sure to include a "diper raffle" for a chance to win a wine and cheese basket. Everyone brings in a pack of diapers for a chance to win! The more diapers the more chances!
Some original ideas for baby shower invitations include light pink tone cards. You may want to include an ultrasound picture, due date, and thin, stringy ribbons on the invitations. Date, time, and location are also needed.