You can buy Brandling Worms at some fishing shops, or on line.
It is not difficult to find them in the ground (have a Google for list of ways to)
Brandlings are what you get in compost heap, and often considered best for keeping soil in good health.
Whether or not you need them? - Difficult to answer that one. Depends on how well the mix in your raised bed is supplying the nutrients which your plants need. (And some plants drain nutrients much faster than others)
The peat is theoretically rich in nutrients, but other than the black fen-land peat, it takes far too long to break down and release what your plants need.
There is the nutrient content of the added soil, and the peat will make the mix humus-rich. If you are feeding plants, or adding fertiliser, or if you regularly replace mix with soil from garden, then you may find that you do not miss the worms.
Nevertheless, worm compost, ie the excreta of worms eating vegetative waste in the soil, is regarded by many as the finest of all growing media. If you do add worms, you may need to add veg waste to feed them.
With a concrete base, and possibly high acidity due to the peat, and a depth of 18 inches might be subject to extremes of wet/dry, I do not know if that is a comfortable environment for Brandlings.