I have received many of my stock cultures from a Uni collection,ran by a well known phycologyst.This way I can be sure I have the right ID and that a good hardy strain to start by was selected.I try to keep them sterile.
I think people selling algae like those mentioned,also acquired their first stock from scientific collections as well.
I don´t know now,but years ago,FAF
discs were very unreliable,I got a TET
culture labeled as something else,and even two species discs,the allien being Chlorella.