Cells drop from bosses, elites, sometimes chests, and occasional enemies. Timed chambers also contain a hefty dose of them. You are prompted to spend them after each level, but you can break down the door after the Collector and continue accumulating them across levels. If you are thorough and don't spend any, you can reach 100 cells sometime in Stilt Village, just after the Concierge boss. Then all you have to do is die!