The drop sources for the Prismatic Shard are the following:
Each Iridium Node has a 4 % chance to also drop a Prismatic Shard, so it's definitely possible to get them from this source, even tough it is pretty rare.
Mystic Stones can be found in the 100 - 120 range in the Mines, and have a 25 % chance to drop a Prismatic Shard. This makes it a much better chance than the Iridium Nodes, but the Mystic Stones are much rarer themselves.
Gem Nodes can be found at any level in the Mine, and has a chance to drop any gem or mineral in the game, including the Prismatic Shard. It's not a good solution for farming the Prismatic Shard, but you should at least check them to see if you get lucky.
Omni Geodes has a chance to give you a Prismatic Shard when opened by the blacksmith, with a 1/250 probability. The game also has a minimum on the number required to open before getting a Prismatic Shard, and the shard has a 0 % drop chance until you have opened 15 Omni Geodes.
Enemies in the 100+ levels of the Mine can drop it, but only from Mummies (0.1% drop chance), Serpents (0.1% drop chance), Shadow Shaman (0.05%), and Shadow Brute (0.05%).
After you have reached the bottom of the mines, any enemy in the Mine can drop it with a 0.05% drop chance. This means that it's best to farm very easy enemies as fast as possible, and I would suggest going for the Dust Sprites in the 40-79 level range. (This is only for patch 1.3.27+, so right now this method is not available on all consoles).
Fishing Treasure Chests can drop the Prismatic Shard, but no one knows the drop rate. However, it is very rare from this source as well.