Former tournament player, where's the best place to buy collections?!

I went to r/yugioh101 and looked for archived decks but couldn't find anything more than one deck. I figured you guys would know! I played in a couple tournaments and did some traveling for the game back in the day, I just happen to finally be in a place in my life where I can return to playing and I'm looking to see if you guys have any tips or markets to best buy large lots of cards! Thank you for your help!