Catalyst sources
Solitude Crystals drop from Verdantstag and arrive from the Mega code. Bamboo Eggs and Crystalfae eggs are the only confirmed Mega candidates at launch.
Mega candidate rules
Only Mega-eligible monsters should receive crystals. Spending on a non-eligible monster is a hard loss. Check the wiki bestiary before committing.
Timing the upgrade
Wait until your Mega candidate has the right Trait and Mutation. A Mega on a Brave + Harold Dark Hiller is dramatically stronger than a Mega on a filler.