Though it depends on exactly what kind of deck you're trying to build, I think your best bet would be artifact sources, especially if you play esper:

[[coldsteel heart]][[coalition relic]][[darksteel ingot]][[khalni gem]][[lotus bloom]][[paradise plume]]

There are also all of the Borderposts from Alara Reborn. Too bad mana burn was eliminated, this card was too much fun:

[[spectral searchlight]]