Talk to the scrape-metal guys who roam neighbourhoods looking for metal, they will have some bikes to sell you
Or you can go straight to the source, your local scrapyard
I did this and asked the guy at the front to put aside bikes for me and I paid him $10 or $20 for each bike
After just a couple of visits, I had to ask him to only put aside bikes that were in relatively good shape because there were too many showing up at the yard
I lost count of how many bikes I bought and fixed up and sold on Craigslist, it was well over a hundred
Old mixtes and 3 speed step thrus were my bread and butter