Where to look for air parts?
 

I'm looking for two parts that were sold as GMC accessories across the line but would be far more common in heavy duty trucks. Mine's only a 2500/3500 so it's outside my area of knowledge!

In any event, I'm looking for the air horn "Solenoid Control Valve" and the little air tank reservoir pictured below. I realize I could get any of these off eBay in modern form, but I'm hoping to find period-correct stuff.

I assume the tank is small, like well under a gallon.

I'm more asking for "where to look" than "does anyone have these", as I doubt they're very common, but don't know where to search.

Thanks!
Dave