I'm trying to get into Yu-Gi-Oh! Could you help me out?

I used to have some fake cards back in the nursery and elementary school, but I didn't really know the rules, so I just kinda collected them because I loved the monsters on it.

Got 60 new original cards today from a Vinted seller but they don't have more cards and I'm clueless where I could buy some more original cards for a reasonable price. Also did the old(er) cards get rereleased? I remember some cards I'd like to buy; Big Eye, Maximum Six, Pot of Greed, 7 Colored Fish, and a black dragon/hydra with many heads its necks are kinda squarely-shaped and spiky (I didn't find the card of it thus I don't know the name)