While Mana Plants have excellent per tile efficiency (well, assuming you're interested in other things in the set, to max link bonuses), they are tiny. It is hard to build a bulk of mana production on Mana Plants; you will need something bigger to make a backbone of your mana operations. The sets mentioned above are excellent for that, but alas, these are no longer available. A farm of Mana Huts/Burning Pools, perhaps Festival Merchants in the lower chapters, would probably be a target for efficient mana generation nowadays. Supplemented with Dragon Abbey for that peak output when it is needed.