A note is a good idea. But I do wish you could feel comfy asking your mom. Woman to woman, you need a mom in your life. And especially about things like this. So, try to get over feeling like you don't want to ask her things. Once you do, it will feel more normal to do so and you will appreciate having her to run things by as life goes on. Otherwise, a grandma, aunt, older girl cousin . . . all are good sources to help you get tampons. good luck
Maybe leave a note for either of them, if you feel that talking about it is uncomfortable? Honestly, though, talking about it is probably better, and it won't be as bad as you think. They know they have a teenage daughter, after all, so it won't be completely unexpected c:
Be sure to specify small size, and probably the kind with an applicator. Read the instructions carefully.