The Membrum core has a very small chance to drop from any futa enemies (Except the Essence Slimes), You can also purchase it from Multiple futas, Finny at the beach (Highest price), The 5th, at the forgotten caverns (Medium Price), Amanda, after helping her and inviting her over to your Nem'yh house (Cheapest).
The Void's cum can be purchased from The Void's bar, found at multiple loot areas on the game, or dropped from The Forgotten chamber creatures, or Void Essences at the Essence lakes.