Mostly it is because of crashes
Crashes may be caused by:
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Minecraft randomly spawns different biomes. Simply create a map and explore. You'll find a forest eventually.
You can get pumpkin seeds randomly in chests in dungeons in Minecraft.Pumpkin seeds in Minecraft can be crafted from pumpkins in the crafting grid.
Every map has them, as they are randomly generated.
You'll have to find a stronghold. They're randomly in blocks. Edit for updated game versions: They can now be randomly found in Extreme Hills biomes as well.
It can be obtained randomly for higher level enchants (I find I get it about 15 or higher).