Craiglist is your friend. I'm always seeing a kiddie trailer on the local CL. Mostly they tend to be low-end varieties.

Although I bought one (Instep) of these about 5 years ago and have been using it to lug groceries for most of that time, at least twice a week. For me it's a 1.5 mile roundtrip.

I'm actually quite surprised at how long my trailer has survived and shows no sign of packing it in. One important modification I did make is to get a Burley trailer hitch and replace the unit that clipped on the chain stay.