Sold in stores and dropped by mobs, common types of equipment are easy to come by. Generally most common equipment is not that good and only used when nothing better is available.
Better than common equipment, uncommon equipment is harder to come by and is usually acquired through looting mobs. Naturally some is more useful than others, with the best of them being the 'very uncommon'. Fire Kirin server does not have the full drop table, so it's hard to say what can drop and what can't, but most of the 'very uncommon' items are available in the Item Mall (Some do drop in game but others are only in the mall).
This is a list of rare equipment in the game. It is hard to give detailed or accurate information on where or whether these are available, as Fire Kirin server does not have the full drop table open (although many of these are likely not obtainable even on Green Kirin server). I have noted if an item is available for regular sale in the Fire Kirin server item mall (Gudang).
This is a list of the equipment sets in the game and their effects.