Armand Premo asked:
"Looking for a source for Frisco, KCS and CIL Boxcars Kits or RTR Any help would be appreciated."
"That's a wide net. Anything specific?"
"Steam era box car, Single sheath, double sheath or ste[e]l. I need some representation of these roads.1950 cut off Although I would prefer 40[i]sh."
Still tantamount to asking "give me every boxcar kit that can be used to model these roads". Sorry, but you're going to have to do some more homework to narrow down what you want.
First thought that comes to mind are PS-1s. All three roads obtained PS-1s prior to 1950 - KCS in 1947 and 1948, SL-SF in 1948, and CIL in 1949.
These are low hanging fruit if you're willing to wait for Kadee to release these roadnames for their pre-1950 PS-1s.
Personally, I think post-1937 steel boxcars are over represented on most layouts as the models have been readily available over the history of the hobby, so going with PS-1s as representative examples might not be the way to go, especially if data shows a preponderance of other prototypes.