You can buy one from the guy next to the silt-strider near Tel Mithryn, another can be found at the end of Fahlbtharz (near the water stone) and 2 are found on one of the Reavers on Kagrumez. You only need 4.
I have also found a resonance gem in Nchardak during the quest "The Path of Knowledge." It is in the room where you have to use the control blocks to set three bridges in place.
Once you have obtained all of the control blocks in the room and filled it with water, return to the highest point in the room (to the right if you are entering the room for the first time). Step onto the overhang opposite the control block switch and look to your left as you face the switch. There is a ledge that you should be able to reach with Whirlwind Sprint that has another resonance gem.
In Kagrumez, the Reaver that is supposed to have the two resonance gems isn't spawning. When i go down there there are only two reaver thugs neither of which have the gems. Anyone have any ideas or in a similar situation? Is there a pre-req that i missed?