Where are you going? For a cruise or country where good diapers are not readily available (like the US), you will need to either bring enough with you, or have them shipped ahead.
If driving, just bring them with you. If flying you can check a bag of diapers for free. If you don't want to lug them around though, you can always order enough online and shipped to your hotel. (though the hotel may charge you for it). If by boat, definitely breing enough with you.
And by enough, I mean 20% more than what you normall use every day. Vacations usually mean lots of walking around, which means you will be drinking more water. Vacations also usually mean more alcohol too. So you will go through more diapers than you're used to.