I just did a Minnie Mouse theme for my daughter's first birthday in February. And you're right, there isn't much out there. I ended up making her invitations with stamping stuff I found at Michael's. We had her cake made by a bakery - it was Minnie's head with a pink bow. I found minnie mylar balloons at one of the local party places and just had balloon bouquets around the house. For the rest of it, I used plain pink napkins, plates, cups, etc. and also used some pink "1st" birthday stuff they had at the party place as well.
For the party favors, I bought the Minnie ear headbands that were online at the
Disney Store when I had a sale code (they ended up being about $5 a piece, but that's all I bought). If you need Mickey ears for the boys, I found those at BirthdayExpress for $5, but I really didn't like the quality of them after they arrived. FWIW.
The party turned out really neat. I do have to say, though, that the invitations were time consuming!!! HTH!
Oh, and I dressed her in Minnie clothes from the Disney store... Very cute!