You can get them at City Hall on Mainstreet, you can also get them through birthday buckets that you can purchase at certain Disneyland restaraunts. The buckets contain some disney toys a button etc. Its my moms bday when we are in disneyland in June as well, so I am definately going to City Hall and getting her a button.
It seems they are available a lot of places. My son who turned 5 during our last trip ended up getting buttons at the WDTC, City Hall, and I think he got one with his birthday bucket from the Lilo & Stitch breakfast. He'd only wear it for a little while and then wanted it off, so every place we went to that we'd planned on doing for his b-day gave him another button. I think we came home with about 20 buttons for different things
Last time I went there for my birthday they only had birthday stickers. Hopefully they have a good supply when you go. Also the guest services booth right near Buurbank Cones in Cali Adv. has them as well.