They make a hose that attaches to the tub spout (push on type)
I see a lot of these on old buildings (100 years +)
Now for the bad news they are illigal as they can cause a back syphonage if left un attended and in a full bath of dirty water AND someone shuts a valve to the riser AND opens a faucet on the same line (riser) a floor below yours.
Having said that go ao an OLD hardware store and they may have it. Good luck in your quest