Mort myre fungi can be obtained in Mort Myre Swamp. It can be used to fill a Druid pouch or used in the Herblore skill to make super energy potions.
It is obtained by casting Bloom with a blessed silver sickle (created during the Nature Spirit quest) near rotting logs. Each cast of the Bloom will drain 1-6 prayer points from the player. Prayer drain is completely random and unaffected by the player's Prayer level or how many items are actually made. Alternatively, they can be obtained as a drop from certain monsters or by chopping sulliuscep mushrooms.
Collecting Mort myre fungi to sell is an effective way of making money, as they are valued by players due to their use in the Herblore skill.
For an exhaustive list of all known sources for this item, see here
Three fungi are always dropped ONLY if the Bloom spell is used while being attacked by a Swamp Crab.