You can find them in random drops (in containers that usually drop trait gems) though upper craglorn. Obviously it will take a while to find some as they aren't exactly common.
Check your higher level guild trader area. I and many others sell them in Rawl'ka in Reapers March at $25k per item or $5k if you provide the nirn. There are two options to get the items with nirn traits: complete the Craglorn quest (many V14 lvl quests) or simply buy them from players. Buying is better option as completing the quest gives you one random item. Once you have one nirn item you can do a nirn for nirn item trade for research. To get the nirn gemstone you can either run around farming blacksmith and woodworker nodes while lvl VR 6+ mobs walk around, or complete top level crafting writs. The writs will sometimes grant surveys and associated higher content nodes. Nirn is much more common in those nodes and can drop from clothing nodes in addition to blacksmith and woodworking. I find the nirn drop rate from a writ survey is 50% with some surveys granting 2 or more nirn.
Also, nirn buffs were previously calculated on your entire base resist, which as noted above caused 2 pieces of gear to hit the soft cap for spell resistance. Developer stated this was a bug and plans to change calculation to be based on that single items resist. This will affect the crafting economy as either players deem it too expensive to buff with this trait, or demand will increase since more items are needed to reach the soft cap.