In terms of comfort, I suggest sticking with any kind of underwear that's made with around 95% cotton, because cotton is a soft, breathable fabric. I don't know about finding the most comfortable fit, but it might help to look at reviews online for certain styles of underwear to see how comfortable each style is. You can also try referencing your pants size/looking at size charts to make sure you get the best fit for you.
For getting access to diapers, that's a difficult one because we have to figure out how you can 1) buy diapers without letting your mom know what you're using the money for, and 2) discard the diapers without your mom finding them and getting curious/confused. I would maybe suggest finding ways to indulge in this practice without buying diapers because it's free and discreet. Otherwise, maybe you could try
cloth diapering (using a flat cloth, folding it around you like a diaper, and securing it with a pin)?