Relic collector seeking assistance

I've worked my way through all the HW relics and have collected most of the Stormblood relics, ofcourse I am working on the new Shadowbringers relic, I would really love to collect all the ARR relics - there's just one problem. Grade I to IV materia and the 75 Alexandrite needed to 'meld' it. The Alexandrite's not an issue, however, obtaining combat materia that no longer holds relevance in terms of stats is, to my mind, impossible or, at least, impractical.
Does anyone know of a way I can obtain low level materia relatively easily? I did think, at one point, that we should just be able to buy I - IV materia from a vendor now, thoughts?