I've searched the internet far and wide and can't find one. I've read they exist but where?  

BTW it's my first revolver. I picked it up for its cool historical factor but I don't as a matter of course keep .357 ammo on hand. I've pretty much consolidated my pistols to 9mm so I have plenty.

You can buy a helluva lot of .357mag ammo with what it will likely cost you for a Manurhin 9mm cylinder, and have it fitted.  

Revolvers aren't like autos, in that much of the parts have to be hand fitted.  You can't just buy a 9mm cylinder, throw it on there and start banging away with 9mm ammo.  It doesn't work that way.  It has to be fitted to the gun, and it's getting harder to find people that I'd trust a Manurhin to here in the States.  It's one of the reasons some companies dropped revolvers, or switched to more modern designs that took less hand work.  Even with those, it's getting harder to find competent people that can work on a wheel gun.  With something as well put together as a Manurhin, it's going to be a short list indeed.  

I'd recommend just buying .357Mag/.38Spec ammo and enjoy the gun.  It's just one revolver, so it's not like you'll have to stock anywhere near as much as you are for multiple 9mm's.  If you find a 9mm cylinder in the future, jump on it by all means.  I would just buy up some .357 in the meantime.

You could send the 357 Mag cylinder off to TK custom and have it converted to 9mm.  You could still shoot 38/357 in it.  Just not +P ammo or any 357 Mag over ~34,000psi.

OP could get a single stage reloading press, dies for both 38/357 and components.

Way cheaper than a 9mm conversion and more fun.

I wouldn't convert it.

Manurhin stopped making firearms a long time ago and Chapuis Arms in France is now manufacturing the MR73. I expect that you pay over $1,000 to buy one, import it, and have it fitted.

There's one on gunbroker right now.

My understanding is that the MR 73 was designed for heavy use with Magnum ammo and is extremely well made.  Enjoy yours!