Just an Xbee from Digi or Amazon or Ebay. and you'll need the USB adapter board. With my Setup (C7, C5, C4) it seems the Xbee S2C unit works best and has the least issues, despite owning the Xbee 3 w/ Zigbee 3.0. Maybe the Xbee3 pro might work better with the C8 and Zigbee 3.0?
If you visit the everything Xbee thread it has links
NB- be certain to get the Xbee that supports Zigbee, as there are many models
and also watch the mount options, as you don't want to solder the Xbee to the adapter. I bought the one with "legs". Also watch the antenna type, I got the RP-SMA antenna type
And lastly, DAM the price has gone up 100% +
