What do you mean by portable? Do you want one that is easily relocated? Or do you want a true portable, meaning battery powered?
There was a post a while back looking for an Airprint capable portable printer. The consensus was that there was no such thing. Maybe someone has stepped up in the last 4 months.
Anyways, there are some pretty small printers that are Airprint compatible (with wifi). But they require AC power. Depending on your usage, you might be able to get away with something like that. I think it was the HP Envy 100 or something like that.
The key would be for the printer to be able to set-up it's own ad-hoc network for the Mac/iPad to connect to for printing.
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