Some buildings have a quasi-official shop which regularly do the labels for all the apartments. The advantage is that they know the exact type of label required (e.g. we have one for the mail, one for the external doorbell and one for the internal doorbell, and they are all different).
Your landlord should know, but you can always ask your neighbor. Unless it's an isolated house/mailbox, then you can probably just order any label.
I ordered it 3 months in advance, but they were still not there when I moved in. In fact I'm still having self printed labels. The good part is that I haven't been charged yet for the official labels.