Most hobby stores have to buy their inventory from the distributors. Based on that, if an item
doesn't move then they will not order it to sit on the shelf. This also follows with the distributors.
With that in mind, they will not normally split up a servo set because they cannot sell it to you
out of the inventory and the followup labor to replace that servo in the set.

If it is a local store and not a chain then they are more likely to so this but most will not.