I get my diapers at mydiapers.com 92 diapers for about $60. But they do not have plastic pants sorry, just diapers , I use two diapers at once and put duct tape on them so I can use them like pullups and if I wet them I just take off the inner one and throw it and I still protected till I get home.
Just type plastic pants into Google - many sites will appear. I have never bought them for a 14 yr old but I I have for a 7 yr old and know they make them in large enough sizes.
You can get the Gerber flat cloth diapers, 10 to a package, in the 24x27 inch size at Walmart or Target. They can be sewn together around the edges to make one diaper out of them or there are websites that sell teen and adult cloth diapers. My daughter is 16 and I get her plastic pants from babypants.com in adult sizes.