You can buy the Bubble Belt from the Clan Shop when you defeat 16 marks. At 24 Marks (Paragon of Justice) it disappears until you achieve the rank of High Gaurdian (28 marks). After that it is always available.

You recieve a Bubble Belt as a reward when you defeat the Antlion mark. There is also one available in Barheim Passage, but I believe you need to have a Diamond Armlet equipped in order to get it.

Selling Battlewyrm Carapace x2 and Adamantite x1 will also give you a Bubble Belt.