Unless magic is consciousness-based, there should be plenty of plants or other evolved organisms in the biosphere already harnessing this magimaterial. (New coined word?) Since magic specifically responds to organic material, and can be consumed by players for various uses, it seems likely that various organisms would have developed to use the mana in more specialized ways than players.
How the creatures gather the mana probably depends on the world. If mana is produced automatically in small amounts by living creatures, then some options might be enormous glades covered in a single moss or fungus (a la Pando that maximize surface area in a distributed way. Possibly they even all funnel that mana to one central area, which is one way to explain why something exists in the wild that would need massive stores of mana to exist.
Alternately, if it exists outside of the creatures, then enormous plants might form to catch it in its travels. For instance, if the mana were to travel in currents up in the sky, you could get giant floating things that, again, maximize surface area to catch as much mana as possible, and then maybe use some of it to help stay afloat.
Regardless, if creatures are involved, you'd probably end up with a food chain: herbivores consuming the equivalents of photosynthetics, predators chasing down creatures with greater mana stores than them. However, going too far down that line may make the resource too mundane. Possibly, using the material requires some rare resource, or for whatever reason it can't be used to power cells -- this would make it less useful, and hence less likely to develop creatures that use it for more uses beyond what the players are using it for.
Alternately, make all forms of the magimaterial extremely uncompressable: you simply can't make it denser than some arbitrary number designated as 'small'. Combined with the inefficiency of energy transfer in predation, this might well exclude from existing any more than a few 'gremlin' species that consume the magic moss species, since a creature that needs to rely entirely on mana to survive would need to be excessively large to store it. That said, that might be another idea entirely requiring worldbuilding, and in any case would provide a hard limit on how much mana the players themselves can store.
Alternate, far weirder idea: The mana is itself 'alive', in a way, or can be made to be alive -- and conscious. Thus, magic from players might be fine because their will somehow makes their mana do what they want, but storing mana outside a body could require 'awakening' a different bit of mana and telling it to form a bubble around the stored mana. Again, though, that feels like it'd be pretty far rom your original idea.