good potty for a heavy tall kid?

Hello wondering if anyone could recommend me a potty for a larger kid (my 18 month old is 36 inches tall and weighs close to 40 pounds). I pulled out his older brothers old potty the other day and it is not happening!!! The seat itself and the pot where the pee is supposed to go into, are sooo small for his size that his parts aren't even close to being over top of where the pee should go.(i hope that makes sense). We live in a two story house and the only bathroom is on the second level and the living area is on the main level so we need an actual potty not a seat that goes on the toilet, and we are in Canada so we're slightly limited in the stores we have here, (pretty much Walmart, babies r us, sears....) any ideas?