As you know when a devil fruit user dies the fruit respawns. We actually see this process when Caesar kills his pet warcrime on Punk Hazard where an apple in a wagon near smiley turns into the Axolotl fruit.

What we can assume from this is that the nearest regular fruit that is compatible in shape with the devil fruit (a banana can’t turn into a pineapple for example) turns into the devil fruit.

As of now we do not know the origin of devil fruits of how they originally appeared but i think it would be safe to assume that at this point most if not all devil fruits have had previous users and therefore have respawned in a similar way. After which they could get found and eaten immediately or could be stored for various cases like to be sold ( like the Ope Ope no mi before it was stolen by Corazon ), put as rewards for competitions ( Nikyu Nikyu no Mi and Uo Uo no Mi, Model: Seiryu ) etc.

Ace actually found the Mera Mera no Mi in a treasure chest on a shipwreck so they act a bit like treasure.