1. Decapitation rune. Each level of the rune increases the chance of the players head dropping.
2. PWarps and Sellshops. Some people sell their heads at a sellshop so you can look around for that too.
3. Ah, bidwars. Same idea for PWarps.
Hope that helps.
Little late - would recommend making sure you claimed rewards for achieving the island milestone and then waiting (30 mins) to see if your island upgraded. If it didn't, you can always open a ticket for staff intervention.