Introduction
GCU Grey Area discovered that the rare Mk3 Particle Repeater is offered in exchange for a "Spacefly Egg Collection", which is crafted from 5 "Spacefly Eggs (authentic)" items. In order to find the latter, think about Spacefly egg lore
Or alternatively, the following spoiler can help.
Note that other items with Spacefly in the name exist so take care to be looking for "Spacefly Eggs (authentic)"