This is the original poster and I just wanted to come back to thank those of you who replied. This is really helpful information and I feel better about the situation. We're going to go Fergusons or Plumbing Supply Plus and get this taken care of.

To the poster who said their plumber does a lot of this for them, I hear you! We like our plumber - he is responsive and affordable - and he knows our neighborhood well. But there's an element of DIY to a lot of his advice that has left us as real novices and know-nothings a bit flummoxed. I would happily pay more to not have to worry about this stuff.

Thanks again to all!

Ferguson is the same as build.com
Call and they will help you. They helped me pick out a mixer kit.
But most kits at Home Depot should suffice.

Plumbing Parts Plus is the place to go. They have both a real plumbing supply desk and a small showroom with actually helpful people in both section.

I have always had the plumbers either supply this stuff or point out my options in a catalog and then I choose and they order it. Maybe I pay more but it seems worth it.

Is the valve accessible from the back? It sounds like he wants to replace the valve -- "ensure mixing valve was copper" -- and that usually requires more access than you can get just by removing the trim.

If that's the case, you pick out a mixing valve and the trim to go with it.

I like the Temptrol valve by Symmons, it's simple and reliable. It's been around for decades so you know you'll always be able to get parts and new trim for it. On Amazon:
https://www.amazon.com/gp/product/B07RF9GQ7W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

There are different models for a shower-only and a shower with tub.

If you Google "Symmons Temptrol Trim" you'll see there are lots of options for trim.

If you walk into Ferguson in WI early on a week day they should be able to help. I had an appointment a few weeks ago at 10 AM - but could have gone earlier and no one was there except for me.

before you go - take pictures of what you have now so it is easy to show and the person at the showroom can translate your needs.

I have heard Sommerville is better than Fergurson - but I have no personal experience.

Go to Fergusons or a dedicated shop like Plumbing Parts Plus in Rockville. Make an appointment with a salesperson and have them walk you through this like you're a 5yo learning to read.

Unless your plumber is going to open up the walls, you need to know what kind of valve you have in order to buy the right stuff that will work with it. If you're opening up the walls, you can choose whatever you want. One of the key question is whether you have/want thermostatic or pressure-balance valve. But even among those choices, a thermostatic valve from one company may not work with the trim kit from another company. So it's important to have someone who can guide you.

I did 3 bathrooms in 2021 and the shower stuff is more complicated than it should be. Good luck!

Hi. We are relatively new homeowners after many years of renting. Our upstairs shower, which had quite old trim, wasn't getting hot. Plumber said he could replace the trim for us at the same time he fixes the problem (which he pinpointed). He told us to choose trim and ensure mixing valve was copper and went with trim, then he'd do the work. We went to Home Depot and stared at trims and valves: no idea what to do/how to match and no good help. We went to the Kohler Store in Bethesda and were roundly ignored. Now I am looking up plumbing supply stores. I found the Ferguson Showroom but the reviews are abysmal, there are no appointments to be had, and I fear just showing up without one.

Folks - is it really this hard? Are we missing something? Where should we go to buy a shower trim and associated parts that all work together and with our plumbing? I don't understand why our plumber can't order associated valves etc. once we pick a trim set we like (which I think we could do online). Should we find a different plumber?

Any advice would be appreciated. This is taking way too much time and energy for what seems like a small thing, and I fear it's our inexperience getting the best of us.

Thanks so much!