Just look for "ABC*" Barnacle sites in the Canonn spreadsheet, those are ripe every cycle and refresh during a relog. At least the one I have been to some times.
Maybe I can figure out on which planet it is, but I'm right at the office now and so...
Edit: a
Link to the Canonn page with the Barnacle list, but I think I had another spreadsheet in mind as there ist no reference to the cycles on this page !??
So just check some of the Pleiades xxx planets in EDDB if there's any Selenium to find. "My" system was/is Pleiades Sector LN-S B4-3 , Planet C3, a site with two Barnacles and 5 MAs per run. Can't remeber at the moment if I found Selenium there.