There are several USB MIDI keyboards available today that are fairly inexpensive. Unfortunately, they require tethering themselves to a PC to do the actual audio synthesis through a USB cable. It would be preferable if you could wirelessly pair the keyboard to your smart phone or PC, or even better yet, do all the audio synthesis right there at the keyboard without a PC at all. 

The PIC32MX695F512H chip that we used on the TRAXMOD project has support for USB On-The-Go, which means it can act as a USB host to the USB MIDI keyboard. Then it's just a matter of converting USB MIDI data to plain serial UART format for a Bluetooth module to transfer over the air. If more ambitious, the PIC32 could be used with an audio DAC to directly synthesize audio without a PC. 

RN-42 short distance, low cost Bluetooth serial module.
RN-41 longer distance, higher cost Bluetooth serial module.