A Thargoid Barnacle is an organic Thargoid structure that extracts resources from a planet and converts them into Meta-Alloys, a key component in Thargoid ships and technologies.[1] The main structure consists of an organic growth with a central, ribbed peak surrounded by Thargoid Barnacle...
elite-dangerous.fandom.com
Alternatively you can buy them from some people's fleet carriers (seem to be stupid prices though):
The companion site for Elite:Dangerous with the game database, market data, trade routes, outfitting, engineers, blueprints, crafting, Thargoid war, squadrons, logbooks, galleries, galaxy information and much more...
inara.cz
I picked up the 10 I required for the thargoid bobblehead by scooping up meta alloys at one of the thargoid titan ruin sites.
P.S. I forgot to mention - I bought the one I needed for the engineer Farseer by going to Darnielle's Progress in Maia (that is what I was told to do when I started) - I just looked on INARA and don't see MA listed so don't know if you can still buy one there.