What is the best printer to buy to use with an iPad ?

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Any AirPrint printer will work with your device. See > http://support.apple.com/kb/ht4356

One that is AirPrint compatible. Check the listed ones here: http://support.apple.com/kb/HT4356?viewlocale=en_US&locale=en_US

I have a HP printer that works without any problem, but you can also look for other printers

I am a huge believer that Brother's Printers are by far the best printers for Apple products and I have tried them all!