Most commercial planes have changing tables in the lavs these days...even the smallest 35-seat jets have them! They're not well-marked and often go unnoticed, but they're usually there. If you're unsure, ask your flight attendant for help.
I strongly encourage folks not to change your babe in the seat unless there truly isn't another option, though....consider that the aircraft seats are everyone's living and eating space. I know I don't consider my wee one's poops to be offensive, but I'm sure the stranger next to me or the next person to sit there just might. You probably woudn't change her on a table in a restaurant, and likewise it's uncool to do it in the cabin of the plane.
I can't tell you how many times I've found a dirty diaper in the seat back pockets, too! I understand how it happens...the baby needs changing, it's done in the seat (or -gasp!- ON the tray table!). Mom sticks the dipe in the pocket and forgets about it...and leaves it for the cleaners or FA or the next passenger to find. I had one poor passenger board the plane, stick her book in the pocket, and pull it out to find it covered in poo from a previous passenger's overloaded diaper. The cleaners don't always have time to check every single seat back pocket on the plane during a quick turn-around...so it's not terribly unusual.
I've had lots of passengers try to hand me their diapers to throw away, as well. I'd love to help, and I have, of course...it's tough when you're sometimes stuck in a seat and managing an infant. But the best thing is to throw it away in the lav...the people handling your food won't also be handling poo, and it won't be stowed in the galley with food either.