Find a mangrove tree in a mangrove swamp biome
Break mangrove leaves to get mangrove propagules
Wait for mangrove propagules to fully grow into saplings
Use bone meal on mangrove propagules to speed up growth
Collect mangrove saplings when the propagules mature
Trade with wandering traders if they offer mangrove propagules
Use commands in creative mode to obtain mangrove saplings or propagules