Got mine from a double Barnacle in the California sector. Needed Meta-Alloy for Farsser anyway, and collect my Arsenic in the mean time, and cramp in a bit of sightseeing. It's still not plenty, but at least when it pops up it's a whole ton. Shoot every single bud on the Barnacles, collect, log out, log in, and do it again and again. One harvest of the double Barnacle yields a couple of tons of Arsenic. I spend a little more than an hour there and got over 15T of Arsenic. It's just that getting there takes awhile. But not too bad for my 26 Lys jump range 'Conda. Took about 40 minutes to get there and navigate to the correct coordinate.
And, you get a lot of Sulphur, iron, and stuff that come in handy for grade 1 upgrades. With those stuff, I pretty much breeze through grade 1 and 2 upgrades for most Engineers with my other already collected materials/data without mining at all. One trip to the Barnacle and you pretty much get a free grade 1 pass, and more. In fact, I am almost through all the Engineers to the highest grade (but still have a lot of ships to upgrade, a fleet of 27 ships is gonna take awhile), and I still have some Arsenic left. Quite a good deal.