I find the best way to get free things is to let people know you're interested in those things. I recently took over the local bike hire, and now everyone knows I enjoy repairing bicycles. I've had multiple people approach me to get rid of their old bicycles. That's how I got 2 of the trailers. This presumably works best in places with stronger community.

Otherwise, I think keeping an eye on local buy/sell websites (eg facebook/gumtree/freecycle). I think you're likely to find an old child trailer, which you can remove the cover and sides from to get a trailer base. Maybe try putting "wanted" posts on these websites as well, as people might not realise a torn or rusty child trailer is worth anything.

Theoretically they should be rather easy to make from scratch, though I've not done this yet. They are really just a frame with small bike wheels and a hitch.