I don't think you have much chance of finding a suitable frame just laying around loose. I have never seen or heard reliably of one and have not seen a custom gun like you are talking about made other than by stripping down an existing revolver.

That said, there have been some fixed sight S&W .44 magnums made. Barrels are short and would have to be replaced to get the style you want.

A fully equipped and skilled gunsmith could convert adjustable to fixed with welding and machining. H. Bowen has done it - he once faked up a Redhawk to look sort of like a 1917 - and there was a shop in my area converting Ruger Blackhawks to look more like SAAs long before the Vaquero came out.