How do i ask for a bra if my mom doesnt think i need one?

I think I need one even a cami bra bcuz im developing but my mom thinks its weird to wear one at 14 she dosent even wear hers all the time and i really need it its embarassing at school and im the only girl in my class without one. She dosent want me to wear one im way to scared to ask should I sneak one? I think she would find out. Please help!