my friends set up has an inline silencer on his ducting, it is quite bulky....
looks kinda like a big version of a baffle box that you would find on a car exhaust, but its made specificly for ducting.

same principal i guess

i would think if your hydro shop cant help you out, you would have to look at industrial air/con suppliers but there are solutions out there.
if you could figure out how they are constructed, im sure you could make a home-brew one without too much hastle.

hope thats of some help