How do i get undead bait in free roam red dead redemption?

well i saw these two guys with undead bait in multiplayer free roam and it was like dynamite like when they aimed it it sounded like they lit a dynamite but when they threw it it did the same little splash of odor that undead bait does and it looks like undead bait too i asked them how they got it but they just killed me and ignored me... does anyone know how to get this????