Our 150H was originally from Minnesota. It came with (I guess) a home made winter baffle kit. It is two separate pieces of aluminum with the appropriate opening, etc. It screws to the front of the cowling. You can see the small holes in the top cowling in the attached picture (ignore the chipped paint!). I don't have the baffles here, but they have been laying in our hangar for going on 14 years. PM me if you want to pursue this further.
I believe the one for an "H" won't fit an "M" cowl which is the later "spacer" cowl. Marty's tip for the Canadian Aerofairings site is where you will find what you need.
If you search my profile you'll see that I made a posting on 12/10/13 titled Cowl Block Plate (winter flying), just like you I was having difficulty finding a reasonably priced pair of used plates so I ended up fabricating my own which you will see in the accompanying photographs. Another good resource for information is from Cessna titled Winterization Kit number AK150-93H dated March 17, 1980 this gives more detail of the entire kit that's a little better than the IPC (illustrated parts catalog). There are three other major components that make up the kit if your aircraft did not come from the factory with it installed which consist of the Y pipe, carburetor heat box cap and the T assembly on the firewall. Hope this is helpful to you if you need any additional information or have questions just let me know and maybe we can get together on the telephone.
Note: Since I originally made this post I was successful in finding a very good used set of baffles, they are hard to come by used.
Well reading these posts I now realise I have the winter kit fitted to my plane minus the baffle to close off the intake, not suprising as my plane was sold new into northern Alberta. Very COLD....
I?m presently repairing/manufacturing new baffles and replacing all the scat hose.
Craig, thanks for the info re. the Cessna publication, now I will make up the baffle to close the air intake, as well as getting the decals made......
As all of this stuff is off the plane at the moment I will photograph it all as I reassemble over the weekend and attempt to post the pics here.