Bubble Belt is an accessory in Final Fantasy XII that gives the user automatic Bubble status, doubling their current max HP and immunizing them to Disease. Though the menu has 9999 as the maximum it can display, the HP can be higher under Bubble.
In the original PlayStation 2 versions, the Bubble Belt is for sale in the Rabanastre Clan Provisioner shop at the market when the player has achieved the rank of Riskbreaker or High Guardian in Clan Centurio. It is also a treasure at Barheim Passage's East-West Bypass (50% chance of spawning + 2.5% chance of containing Bubble Belt, Diamond Armlet is required), and a reward for the hunt for Antlion.
It can be made in the bazaar as the "Chain-link Belt" by selling two Battlewyrm Carapaces, and one Adamantite. It can then be bought for 17,820 gil. Battlewyrm Carapace is available from aevis-type enemies (the easiest is probably the Skulwyrm in Zertinan Caverns), and Adamantites are available from the Adamantitan on the Cerobi Steppe.
In the updated Zodiac versions, Bubble Belt is no longer available in the clan shop, but sells for 9,000 gil. It is a treasure in Henne Mines' Crossover C (west end; 25% chance of appearing), Feywood's Ice Field of Clearsight (near the middle of the area; 100% chance of spawning), and Pharos - Subterra's Abyssal - North (70% chance of appearing; 80% chance of gil; 50% chance of being Bubble Belt when it is an item). It is a rare steal from Trickster in Trial Mode Stage 58.
It is still made in the bazaar as "Chain-link Belt" with the same requirements and price as in the original, and it is still a reward for the Antlion hunt.
Instead of being sold in the clan shop, the Bubble Belt is available in the Hunt Club shop if the player gives 25 trophies to Stok. The Bubble Belt can then be bought from the Shifty-Eyed Merchant's store for 47,520 gil.
Bubble Belt is one of the most useful and valuable accessories, as thought the effect can be replicated with the spell of the same name, as the effect will not lapse mid-battle, and wearers are revived from KO on double health, which doubles excellently with Arise or Phoenix Down with Pheasant Netsuke. Having double health is highly useful in some of the hardest and longest battles, such as against Rank VII hunts, Yiazmat and Omega Mark XII. Bubble Belts can also be used to immunize the wearer against Disease.
It is worth going out of the way to procure the items to make it in the bazaar as early as possible, and in the Zodiac versions find the chest in Feywood, as it always appears. The one in Henne Mines is also fairly easy to find, but has only a 25% chance of appearing. In the original version, it is best to go buy it from the clan shop as soon as the player reaches Riskbreaker. Without the belt, the player can use gambits for similar effect with Self:Bubble and Ally:Bubble.