As Orkekum already said, fly to
Maia, land at
Darnielle's Progress, and buy your meta alloy there. It is a bit of a long trip depending on where in the bubble you are (400 LY or so), but it is the fast and reliable option.
The alternative is to visit one of the known barnacles and hope that they have not been harvested "recently" and are not surrounded by defenses. There are about 30-40 sites in the Pleiades nebula, check the
Canonn Science Team web site for their barnacle location and status list. Since this is the nearest source, everybody uses it, so your chances are poor, and Maia is in the Pleiades nebula, so you do not save anything trip-wise.
Additionally, barnacles are persistent planetary points of interest and as such, they do not show up on your ship's scanner, so you have to find them by coordinate and then by looking out of the window. This is a bit of a challenge. If you want to give it a shot, fly over to Merope 5c, and visit the first barnacle ever found at -026.3496, -156.4044.
At the moment, I am aware of exactly one unharvested and undefended barnacle, and that is in Witch Head sector, which is about 800 LY away from the bubble, and surrounded by permit-locked systems. You will need a jump range of at least 18.7 LY with your meta-alloy on board to enter or exit Witch Head sector. See
here for details.
More unharvested and undefended barnacles might exist in the California Nebula, but that is even further away, 1000 LY from the bubble.