It can feel embarrassing to ask your mum to buy you new bras for the first few times. There are a couple of ways to go about it without feeling too awkward, which is never fun.
How to ask your mum to help you find new bras
The most important thing to remember is that your mum will have been through exactly the same thing when she was younger. It's most likely that she will be completely understanding and be eager to help, as it's very important to be comfortable.
If you find it too hard to ask her face to face, you could give her a note, or put it in her purse or somewhere you know she'll find it. This would give her time to work out how you can work it out together.
If you really don't want to ask your mum, there are a couple of ways you could get bigger sized bras for yourself.
Buying new bras for yourself
Remember that no matter what age you are, stores will always help you out! If you feel embarrassed to go into a store yourself, you could take a friend who you'd feel more comfortable with.
A lot of department stores offer free fitting services, where they'll help you select bras that fit you perfectly to try on. There's no obligation to buy the bras so it's a great way to see the kind of thing you like. You could even pick some then try and ask your mum to help with the cost.
Alternatively, you could have a look online. This is a bit risky as you can't try the bras on first, but if you check out the website's return policy, you may be able to send them back if they don't fit.
Bra sizes do vary from store to store but you may able to check the exact measurements a store carries in the product information.
Good luck, remember that every woman has been had to work out how to buy new bras at some point in their life so will know how to help you out!