There are multiple ways. On your first kills on the mini bosses in The Roost of the Graveborn, they will drop you 1x Fetid Heart and one Crypt Fossil each, giving you 4x of each of item.
They are also drops from the mini bosses, but they seem to be very rare. So far we didn't get a single one in over 120~ kills in total (30x kills on 4 man)
Inside Forsaken Crypt (the solo spot) is the best way to drop Cryptic Fossils. A high level drops around 2-3 per hour
Forgotten Crypt (the easier, less dense TH spot) drop Fetid Hearts, but since the spot is less dense, I'd expect max 1 drop every 2 hours
Unhallowed Crypt is the more difficult team hunting spot, that require Fossil Crypt charges to enter. Here, we usually drop 1 Fetid Heart per hour, sometimes every 2 hours on average
