In the Shadowlands, there are Nano Crystal Vendors in each garden and sanctuary. You will need the appropriate Key in order to trade with them. The vendors in pandemonium are the exception to this rule. In Omni-Tek Gardens, they are usually on the east outskirt of the gardens, Except from the Garden of Thrak which has the nano crystals in the same place as the Usual Vendors (Down the stairs just right from The Core statue exit).