While 
Resonance Crystals aren’t exactly a hot currency, players still need them to buy the Renown rewards like transmogs and mounts. Well, if you’re finding yourself missing some (and played in Azj-Kahet a while), here’s a fast way to get some.
Kej for Crystals
Halfmanhalfsneaker very helpfully reminded us all that there’s a handy NPC in Azj-Kahet that can trade the local 
Kej currency into Resonance Crystals! And, as zztopar points out, there’s even more stuff coming in patch 11.1 that we’ll be spending those crystals on, like the new rocket mounts!
And if you need the specifics, the exchange rate is 2:1 Kej for crystals in two bundles of 200 and 2000.
You can find Ka’muko in the Weaver’s Lair zone hub, at 56.6, 43.0.
Alternatively, as mentioned by anythinga, each difficulty of the Skyriding races also offer a fair bit of crystals on first completion. So if you prefer flying around to currency exchange, maybe check those out.
