Welcome aboard Ken

I´d suggest that you check out Graham Industries...if you aim for machined kits.
If you intend to machine them yourself, i am only aware of PMR when it comes to steam engine kits, eventhough i would assume some castings is available elsewhere as well.
If you wan´t combustion castings, Martinmodels could be a solution as well:
All Martin Model gray iron castings are fully annealed for ease in machining.Â