Can you wait and buy it when you arrive? NO
A small rubber door wedge is a useful security measure and very handy when you have a bed with wonky legs. It may seem a silly thing to worry about, but a good night's sleep can be essential and a bed that wobbles can drive you nuts!
Its main use though is to stop people pushing open your door at night. Some accommodation has locking doors, some may have just a latch. A door wedge pushed firmly under the door from inside the room makes it hard to even kick a door open, and nobody is going to manage it without waking you.
You and your possessions are most vulnerable when you are asleep-so pack a door wedge. If you forget, or lose it, a bit of wood can be easily shaped with a knife and will be almost as good.