ANDY'S ANSWER: Your options are to 1) Do nothing and just change your underwear frequently. 2) Buy your own panty liners. 3) Tell your mother and ask her to buy you some. I would like to point out that your Mother is a girl and has gone through this herself. This is a natural part of you growing up and nothing to be shy about.
Pick a time when you can talk to her privately and let her know that you are getting close to having your period and need some supplies. She knows you're growing up and I'm sure she is expecting that you will tell her when you need these things.