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I don't think your need to worry about your mom, she wants you to wear diapers it seams so just go shopping with her and you two can pick out your diapees yourselves. As for the reputation thing, well in my view your SOL.
Well, obviously you need to be sneaky about it. Try to find a store that's near you, hop on your bike (or just walk), take some cash, and buy them. You should bring a backpack. And if your parents ask why you're bringing a backpack, tell them you're bring water in case you get thirsty. Also, if your mom recently discovered, you should probably stop diaper-wearing for a while, until she becomes less concerned.
Get good grades in school. Be mature... Then maybe she wont even care. I know that when my parents discovered on my end and asked about it, i just said one word to my mom... "Puberty"
Then, when my dad asked about it, i said "it's taken care of." or something along those lines.
Ye need to make it clear that you are mature, and can stand on your own two feet without whining about it. (ie: Help around the house. Don't bother getting angry with parents. Clean up after yourself.) After that, then hopefully they will respect you enough to not inquire into it anymore, or they will see how responcible you are and come to the conclusion that it's not that important.
I think most parents would rather ignore it. But they can't really do that if they feel they need to be taking care of you all the time. Show you are able to act maturely and responsibly. Then they will hopefully feel less of an obligation to care for you like a kid, at which point they will be able, in their mind, to choose to ignore it.
So make it easy for them mate that is the best route in my opinion.
As for purchasing them... Just make sure that they never have to see it; that would make it very hard for them to lie to themselves, or ignore it. After you make sure of that, it does not matter where you get them from, as long as they are never seen by your parents.