Where can I find an old lower unit of a small boat motor without an engine for cheap?
retired old sarge: https
https://mudmotorkit.com/mud-motor-kits/mini-swamp-runner.html best cheapest motor you can make..
I doubt that you will find a lower unit to match up to a lawn mower engine. However if you have the tools, knowledge and the motor has a horizontal shaft it can be used. Just add a propeller shaft and a stuffing box. Of course you'll only have forward gear as it won't have a transmission.
Find a marine salvage/scrap dealer.
While it is possible, unless you have access t oa lathe & welding equipment, can use both effectively it wont last. There is a fairly high load on the connection & lots of vibration. Unless it is designed & built properly it will fail very quickly. Take a look at long tail setups - may be more do able for you.Be careful rotating exposed shafts are dangerous.
Go to an outboard dealer or outboard wrecker or boat junkyard
Look to see if there is a marine salvage yard in the area where you live. Or go to Craig's List or Ebay to search for old outboard motors.
Talk to some boat dealers or boat repair shops and see what they have.
Do other people and nature a favour and do not put that load of Junk in the River!
Most people in the boating community call it a Leg. my bayliner's I4 connects to the Leg..The leg has a propeller on it and is basically a transmission.
Harbormaster auctions of marine property seized for unpaid debt. Everything is owned by somebody. Found property that later is discovered to be stolen must be returned. Consider yourself as a likely suspect.