Hmmmm. I've been using InStep trailers for the last 8-10 years and never had a problem with tires coming off. And these had plastic wheels too. Might want to take a look at the tires to ensure they are seated properly on the rims and maybe inflate them to a few pounds under the max if you're having these issues.
You could look at the Schwinn trailers for interchangeable parts. I've noticed a lot of parts seem to be the same between these two companies and Schwinn uses traditional metal rims on some of their trailers. You might get lucky.