Ransoto slip on from Amazon... it’s a China made one but it’s worth the money spend. There’s a lot of no name brands of the same one.

Motorcycle Slip on Exhaust system With Muffler Compatible with Kawasaki ninja 400 Z400 2018 2019 2020 https://www.amazon.com/dp/B07D8S618R/ref=cm_sw_r_cp_api_glt_fabc_DRV5NPAKWVPKFJFANW6T?_encoding=UTF8&psc=1

I payed 60$

It’s ridiculously loud. I would say even the Harley guys look when I drive past. Without the Db silencer I’m kinda scared to drive through town lol.

https://www.amazon.com/dp/B07D8S618R

This guy SLAPS

You can also type in "Ninja 400 exhaust" into Amazon for a few other options.

Motorcycle Slip on Exhaust system With Muffler Compatible with Kawasaki ninja 400 Z400 2018 2019 2020 https://www.amazon.com/dp/B07D8S618R/ref=cm_sw_r_cp_api_glt_fabc_754QE7HC7S0904RK4JDB?_encoding=UTF8&psc=1 This one you need to get a tool from harbor fright to open the pipe so that it fits.

Kemimoto Muffler Exhaust Baffle Compatible with Ninja 400 Z400 2018 2019 2020 https://www.amazon.com/dp/B07SNSWV1S/ref=cm_sw_r_cp_api_glt_fabc_T04FZC7FY3622QP6D6GN This one needs no modification. I bought both and use this one. Just be prepared to do a return. It seems like their quality control is not as good. But if you get a bad unit return it for a good one and you will be good to go.

Ransoto? https://www.amazon.com/dp/B07D8S618R?ref=ppx_pop_mob_ap_share

Cheap Chinese model but it sounds way better than stock.

Mine's still going strong into its fifth riding season. There's a tiny bit of rust on one of the straps, but it's in a place where you can't see it.

I'm still planning to get a full system some day, but this has been one of the best values on the bike.