Perhaps you could ask a commercial HVAC company about an add-on for your system. You need to also consider repair and storage areas.
Ian does the store have forced air heating? If so after decades of using every console and outboard humidifier on the market (here in the US) and hating every minute of it I recently had a HVAC company in for a home solution.
I'm thrilled too! My furnace humidifier made by AprilAir is keeping 2,200 square feet nearly exactly at 45% regardless of temps in the single digits outside. I bought a unit rated for 4,500 square feet because these companies always lie.... and rarely understand that for wooden instruments, people too, 40 - 50 is a good target range.
It's all automatic, no water to schlep.... no wicks to replace or clean and nothing to ever refill or monitor. This unit has been working for me since November and I am thrilled with it in all respects. It's set and forget too.
Because of the size of the store you may need to employ multiple solutions but if there is a forced air heating system an industrial sized unit, properly installed will at least boost levels most places in the store for you. Once the levels are higher console solutions may get you where you need to go.
My unit takes it's water directly from the hot water heater and atomizes it into the output of the forced air furnace.
One last comment. Our commercial repair shop depends on proper RH to do out work. We would be pretty irresponsible as well if we did not maintain an environment that when someone drops off a $75,000 D-Aquisto and it cracked in our charge we wouldn't be in business very long now would we.....
As such it's imperative for anyone doing repairs in a climate that does get dry in the winter to have a proper RH solution.
Regarding the store owners they have a duty as well to maintain their inventory properly and maintaining proper RH is just one of the many essential things that we all have to do in this business. The store that used to be in our space also ran multiple automatic AprilAir humidifiers on every floor. There is no escaping it and the damage done will very likely exceed the eventual sunk costs of a proper solution.