In Grenada, it looks like most of the time, your guide will bring the containment device (ie>zookeeper) for you so that you don't have to purchase/travel with one. That just leaves you to purchase a pole spear. Grenada also allows commonly available pole spears so you don't need a special one like the ELF tool. You should still measure to make sure it fits in your checked luggage.
2 common size pole spears for newer hunters are the small 24 inch and the 30 inch. Grenada's lionfish have been actively hunted so the smaller 24” spear means it will be harder to sneek up on them than the 30” which gives you that extra distance while still fitting in your luggage. The spear tips are 3 prong paralyzer and come in both barbed and non-barbed smooth. Get the barbed so the LF's stay on the spear better. As you get faster shooting them, you can file down the barbs to make removal faster into the containment device later. Holding a pole spear while the rubber band is stretched can be hard on your hand muscles/slip unless the spear has “finger-ridges” to help hold it. One recommendation to solve all the above is the ZooKeeper 'Sling Spear' LINK
https://lionfishzk.com/product/zookeeper-sling-spear/
They have different lengths, the gripper ridges, removable tips and colors. This is a great beginning lionfish spear. There are cheaper plain pole spears out there, but as you encounter the above mentioned issues, you'll need to customize them. Also if you get a longer than 30” polespear, that may put you in the law enforcement cross hairs suggesting you are targeting protected reef fish by carrying a 5 foot collapsible long spear.(Go on vacation>leave on probation?)
To avoid / when you get stung by the spines follow these 2 suggestions. When handling lionfish on land or underwater, it's best to make a fist with your hand closed. Insert your fist's thumb in to the LF's mouth and press down on the top of your index finger, all while keeping a fist. It's the best way to get a strong grip while not getting stung. If you do get stung, use your dive mask case and fill it with hot water from the outboard motor's tinkle stream and soak your hand in it to help neutralize the venom, hotter is better. 30-45 minutes hot soaking and you'll be back in the water getting revenge.
Have fun on your trip and if you really enjoy hunting them, come visit either southeast Florida or the Panama City Florida areas because they also have plenty of lionfish to be hunted year round, no license required.