Where can i find a good mattress for my sleeper cab?

Grab a tape measure and check your bunk size. Though most trucks are a twin XL. So google twin XL matresses and pick one with the features you need/want. There are literally thousands of choices. For example first one that came up for me was this.

Truck Mattresses | SleepDogMattress Truck Mattresses

 

My Peterbilt has a 66” bunk, and takes a standard twin mattress. I was having very similar back pain to what you described, so I just put a new Serta mattress in mine. Standard twin mattress is a perfect fit. 11” thick, very comfy, $120 (on sale) at Menards.

 

I drove a 2018 Cascadia and for sure it was a twin XL. Basically half of a standard king. Should be real easy to get a new mattress. I'd like to think the company would somehow take care of you.

 

I had a Drift mattress once. They have all springs in them and don’t sag. Even if you sit on edge of mattress it won’t sag or crush. The actual mattress cover seem a little to thin or cheap in my opinion for the price of the mattress. It never ripped or sag. It a very firm mattress but very expensive.

 

Yep, I just bought one on Amazon the other day. $100 bucks for a really nice one that says it's 8" thick but is actually more like 6". I'm keeping for now but the 10" thick ones are available and not much more money. And they're amazing.

 

You can have a custom one made to your liking.
https://www.custommattressfactory.com/rv-mattress

 

Double post.

 

After many years on a thick memory foam I sprung for Tuft & Needle. Little bit better quality. I wanted to go with a known brand. My only complaint about the thicker mattresses is that the platform and stuff around the mattress are built for thinner mattresses so it may need some space filling. I opted to fill the head and foot area with half a pool noodle each.

I've got a Casper at home that's been good as well, though I don't know if they make twin XL.

 

cascadias weird twin XL.

Ultimately since I was a company driver OTR, I just bought 2 layers of memory foam mattress toppers and then added a mattress cover and a fleece blanket as extra layers.

Super comfy, but i also had 5-6 pillows when I was otr as well.

I made it as comfy as I could, because we get F'd all day long on the road. Nothing worse than a bad bed.