As long as you have Demonic Tutor, you have your most important card for any given situation
If you want to play some kind of self-mill thing with cards like Narcomoeba and Creeping Chill in Explorer, this can also be a good addition
Sometimes there are obvious cards or combos that I didn't even think of when I was deck-building, such as Phyrexian Dreadnought, which Abe Sargent pointed out to me would have fit right into my Curie, Emergent Intelligence list a few weeks ago
Credit: Wizards of the Coast Line Cutter FromTheShire: Is this worth reanimating over and over to get another crack at your funhouse
Those include ones with powerful EtB effects, powerful ongoing effects, ones that dominate the board and ones that are really hard to deal with
Online Marketplaces: Websites like eBay and TCGPlayer are popular for finding specific cards or dealers such as PKM HOBBY