I agree. Your bike shop clearly hasn't got a clue ... people have been touring on bicycles for years with loads far in excess of your ebike with guess what? Rims and spokes in wheels ... getting a rim is not rocket science. Calculating spoke length and either buying them or cutting them is not something any half decent bike shop should have an issue with.Take it to another shop. Any bike shop should be able to fix it.
The manufacturer wouldn't be likely stocking spokes anyway. I cannot recall ever having a bike shop go back to the wheel manufacturer expect in the case of some high end rims I had with an unusual spoke shape and size that needed to be matched. For standard spokes they should be off the shelf or cut to size.
Andrew