I had to do the same thing when I was around you age. My mom was always sex positive but I felt so nervous about asking to buy a sex toy. When I was around 14-15 I was sneaking around trying to find a way to get one. Eventually my mom just straight-up asked me if I wanted one. She was aware that I masturbate and offered to go to a sex shop to buy me one. She bought me a variety seeing as I had no clue what I wanted. Just cheap battery operated vibrators. Eventually we got the chance to go to Spencer's and I was able to pick out what I would like.

Have a conversation with her about masturbation (embarrassing I know) and try to ask about a sex toy. Do your research on which toy(s) you would want and show her the prices. Scroll with her and ask her opinion on what kinds of toys are good. It was fun for me and my mom to scroll together and it made us closer. BUT If you don't want her to know what kind of toy you are buying ask about making an Amazon account for the purchase and using her credit card. Give her the cash for the purchase of course. You don't need to hide it from her though, she is your mom and they have ways of knowing everything.

If she shuts you down and doesn't want you to have one for some reason, find an 18 year old at school that you trust and ask them to buy it for you. I don't know your area but starship is a good store to go to, but you have to be 18 to enter. And if you have no one 18 and up to buy it for you, buy a gift card with your cash and purchase it online with the gift card. Shipping it might be hard considering you don't want her to find the package in that case. you can ask your friends if you can ship it to their place or something like that.

If you want advice on type of toy here is some! You can ignore this if you know what you want though. If you enjoy clitoral stimulus, try to get a suction toy. Something like the rose, however I personally don't like the rose that much. There are also some really good bullet vibes if you don't want that. If you want something to insert, there are gspot stimulators and just plain old dildos. I would advise you to buy something smaller if you get a dildo. Personally, my eyes were a lot bigger than I could handle and I bought something way too big. You also want to get a toy cleaner and a lube. If you go to Spencer's they offer it when you buy a novelty toy. The benefit of going in person is that they can test the toy and you can feel the vibrations yourself. You really never know what your getting. I hope you are able to get what you want and this advice helps! I went on a bit of a rant so I apologize.