where to replace popup trailer tires?

One item to consider: Many automotive shops do not have the correct equipment to balance trailer wheel/tire combinations.

Most trailer wheels are "lug centric", while most automotive wheels are "hub centric". This matters when balancing. Feel free to Google something like "trailer wheel lug centric hub balance" for more info.

I had to hunt around to find a shop with a lug-centric balancing machine.

Of course, after surveying a number of pop-ups and many other trailers, I'm one of the few who bothers to balance trailer wheel/tire combinations...