High-level Warframe takes a lot of Forma. You need 1-4 Forma to build any of the clan research weapons or sentinels. You need several Forma to fit all the best mods into any weapon or frame. And the alerts that reward Forma are few & far between.
Clearly the solution to this is void/tower keys; pretty much every tower mission includes Forma in its drop list. Which keys are the most efficient for acquiring Forma, though?