Dodgy Mike's Second-hand Clothing

• Complete the "Cabin Fever" quest to obtain a Little Book o' Piracy and access to Mos Le'Harmless, a pirate-themed town. Dodgy Mike's Second-hand Clothing, a store in this town, sells both pirate boots and a variety of other pirate gear, such as pirate bandannas, pirate leggings and stripey pirate shirts. Purchasing a pair of pirate boots from Dodgy Mike's costs 350 gold coins. You must have the Little Book o' Piracy in your inventory while visiting Mos Le'Harmless, or you will be unable to understand the pirates and purchase the boots.
  
  

Rock Island Prison Evidence Lockers

• Complete the "Rocking Out" quest, then return to the Rock Island Prison and kill a customs officer to obtain a customs uniform. Equip the uniform and travel to the evidence locker room, located on the second floor of the prison. The cabinets containing the customs evidence files can be ransacked with a thieving skill of at least 63 to obtain pirate boots, other pirate clothing items, coins, uncut gems and other valuable items. Doing this grants you 75 thieving experience.
  
  

Pirate Implings

• Once you have completed the "Rocking Out" quest, you can catch pirate implings in the "Impetuous Impulses" activity. Catching these implings requires a butterfly net and an empty impling jar, as well as a hunter skill of at least 76. Once you have caught a pirate impling, you can loot the resulting pirate impling jar to obtain pirate boots, other pirate clothing, pieces of eight or gold bars.
  
  

Grand Exchange

• You can also purchase a pair of pirate boots from the Grand Exchange, located in the northwest of the city of Varrock. If you don't feel like spending the time and effort completing the various quests required to obtain the pirate boots directly, purchasing them from the Grand Exchange can be a good choice. However, you should be prepared to pay an amount of gold coins several times higher than what you would have spent purchasing the item from the store on Mos Le'Harmless.