How could you ask your mom to buy you some condoms?

I wouldn't. Just find a vending machine or buy them from a self-serve checkout so no-one can see what you buy. Also, you can buy them from pharmacies if you are of suitable age like 15 or 16. Don't ask your mom. She will ask lots of questions and if she says yes then she will ask about what you do in the bedroom, how you do it and if you need help. You would die from embarrassment!