I found my first treadle machine (a redeye 66) by looking in the classified ads in the newspaper - found under antiques, but you can also find some under sewing machines. I paid too much for it - I know that now - didn't then. OTOH the cabinet had been refurbished and the machine had already been cleaned etc.
I found the other one on Craig's list.
You can go to craiglook.com and type "sewing" in the what you are looking for box and then your zip code in the area. If that doesn't bring up anything, there's a slider where you can expand your search. I usually make it 100 miles since we are in the boonies and woods.
Sometimes you can find a machine at Goodwill, Salvation Army, Catholic Services, etc. You do have to check them out 'cause sometimes kids mess with them.
If a machine is hard to move it's probably just 'gunked up' with years of oil and grease. Even a little bit of rust can be cleaned off - the Spartan I got the other day had considerable rust on the underside - which I didn't know until I got it home, but I sprayed it good with PB Blaster, waited awhile and then used a wire brush on it (do NOT use a wire brush on the black paint area).
Sometimes the tension knob is gone, or the wire spring is ruined - these can be replaced but it's an added expense.
Belts almost ALWAYS need replaced (generally) as do the smaller bobbin wheel rubbers - they can be cracked and crumbly - but that's no big deal either.
You can also look in "junque" shops, resale shops, antique stores, etc.
Or even an old time sewing machine shop - there are still a few of those around where they have a back room full of stuff.