On the basis of my enquiries to date, I suspect you will need to find a receipt printer with a wireless or bluetooth interface that also has the port for connecting a cash drawer to make it open.
There are other some protocols such as the AirPrint interface and other Printing Apps from the AppStore that will drive a networked printer. But whether any of them work with a receipt printer is the challenge.
Our office supply rep told me about an Epson TMT88V receipt printer that has a wireless interface but it was far too expensive for our proposed applicaton.
There is also a range of Intermec Mobile receipt printers that might work but I doubt that any have the interface for opening the cash drawer.
I suspect there will be more printers in the near future that can meet our requirements.
I hope this helps a little.