If a specific bra isn't available in her size, "sister sizes" can sometimes work. If you go up in the band, you must go down in the cup (e.g., a 34C is roughly equivalent in volume to a 36B). However, stick to her true size whenever possible for the best support.
Buying a bra for your girlfriend is less about the garment and more about the thoughtfulness behind it. By taking the time to learn her preferences and sizes, you demonstrate a high level of intimacy and care. When in doubt, a to a high-end lingerie boutique paired with a beautiful bouquet of flowers is a foolproof "Plan B."
Look for her most-worn bras. You are looking for two numbers: the band size (e.g., 34, 36) and the cup size (e.g., B, C, DD).
When you are ready to buy, follow these "safety" protocols to ensure the gift is a success: