Though most airlines have some supplies on board for nappy changes, these are generally for emergency use rather than intended to be a complete resource, The further from base the plane is, the less well stocked the supply becomes, so you are well advised to take your own provisions. You should be able to find a change room or adequate toilet facility near the departure gate in most airports. Ask the cabin crew for assistance and on some airlines they will prepare a toilet for you, making sure it is clean and so forth. On others you are best to investigate for yourself.
Some planes have a larger than average facility at the very rear.
If you choose to change your baby in the sky cot or on your seat, you will not be the first to do so. Remember to be sensitive to your neighbours, however, and to the cultural sensitivities of your airline. Wrap soiled nappies in one of the perfumed polythene nappy sacks available in supermarkets, and dispose of in the bin in the toilet. There was once an arrogant first class passenger who would do nothing for her infant. Each month has a different fun theme like water babies, messy kids, and holidays.