Well, stealing is generally bad. Its kind of the same thing, you dont have any real medical reason for needing them but your tricking your parents into thinking you have one forcing them to buy something you don't need. Your cheating them out of there money.
I have done this before so I can give you some tips if you actually go through with it. (Mind you I was 12 and very ignorant when it came moraly right and wrong things) Just consider that it is a bad thing to do, even if its your only way. Also, maybe just start a habit of going out for walks. Slowly convince your parents that you want to be outside, WARNING this can lead to a healthy lifestyle which my cause future success as a person (Im not calling you fat, lol! Just joking around)... Anywho...
Start off slow, you going to need to have wet sheets for the first few nights but not in a row. Do it once, then not the next night, then again, then not, ect. Gradually pick it up untill its starting to happen every night. Your parents will definitly notice but hopfully they wont think that there is a huge medical problem (that why you gradually introduced it).
You are at a good age to pull this off because alot of normal kids start to really wet the bed at around that age, due to puberty and such. So it not all that hard to beleive.
Prepare for a trip to the doctor, your parents will definitly do this. Its most likely (due to your age as stated earlier) that you can pass it off as a phase. Which is very good for when you want to stop.
Remember, you don't want this to happen right? You don't know that its a phase that you will get over, you also don't know a proper solution to the 'problem'. In this case, diapers. Your doctor may offer pills but it is up to you to accept them, don't outright refuse them. You are going to 'think' about it for a while to make it a more convincing story that you don't want to take pills. Your going to need reasons for it too, "Because I dont feel comfortable putting chemicals into my body that change things" is a good starter, then add "Besides, I would hate to have to take some kind of pills every night. It would just be, annoying" Dont say inconvenient, because whats convenient is what your trying to get. You don't want them thinking that you want them. Simply say "is there anything else?" Diapers will most likely come up "Its..ok..I guess..." Sort of a dissapointed voice, looking at the floor, you might even turn a bit red, thats all good.
Thats about it. It will get you diapers through fake bedwetting, I assure you. If for some reason it doesn't go to plan you could say that you researched a bit about it and would like some of those goodnite things, they would be a big help and thats what you would prefer. Again, don't sound too excited. Thats how I did it, and any parent that wants to take care of their child will probably fall for it.
EDIT: I doubt any examinations will occur, maybe a urine test but thats about it. It will prove you dont have any medical problems and that is probably just a phase that will wear off eventually.