Theres one on Amazon for $20 that iv been eyeing. I want a case that resembles my Nintendo Switch SkullNCo MaxCarry case so I have a little compartment underneath the steam deck to carry my low profile dock, my HDMI, and my charger, plus I can set my steam deck into the case and use the case as a little kickstand, to play with a controller on the go. There's a few of them floating around on Amazon, some with different features like a built in 10000mAh battery bank or a micro SD card storage slot or a scratch preventing flap.
There's also this JSAUX case that's styled like a small backpack that has a decent amount of storage in it. Might work great if you already have a steamdeck with a kickstand case already. This one's probobly a knockoff of an actual carry case that iv seen somewhere before, I just can't put a name to it but I saw a youtuber advertising this exact style of case before.
If you're not being limited to what you can bring, you can get this giant messanger bag styled bag/box and bring everything you'd ever need for a steamdeck with you. Slots for controllers, probobly enough space to bring several battery banks, maybe a keyboard and mouse if you're going to be gone for a long time. This is probobly OVERKILL or might be good for someone that wants to keep all their steamdeck stuff packed and out of sight when they're not using it.
I just got my steamdeck and I'm in the market for a carry case as well. Waiting till next payday to upgrade from my OEM carry case and I can't wait.