A great option for any deck that falls short of damage thresholds or dedicated burn decks
Endless Atlas is a great card in any mono-colored deck, and Reliquary tower is always good in decks that like drawing cards
But where they do need a lot of help is getting to the point of the game where those threats are castable whether through ramp, or just putting on pressure and defending yourself with low-mana creatures
That monster cant attack, and it must be destroyed during the end phase
There may be others out there, but these are the big ones: classified ads (of which Craiglist is likely the most popular), Ebay, and your Magic social circle (this includes Facebook groups, friends, local gaming stores, and players in your local area)