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Canon and Nikon have manual focus film bodies with long native mount firstparty and thirdparty lenses, with 400mm+ focal lengths. Good, fast (you will need fast with film), 400mm glass is expensive, having tried the budget options I'd suggest don't waste your time with them as they will be disappoining.
You could also consider a canon EF film body + a first generation 100-400mm IS L and possibly a x1.4 if you need more reach. An EOS 1N + the 100-400 can be had for < $1000.
Photo is a tamron adaptall 300mm f2.8 on a canon T90 body shooting Velvia 100F.
