Here is the list from Amazon that shows the latest firmwares. It's the first result on Google if you search for "kindle firmware"
https://www.amazon.com/gp/help/custo...MQC26VQQMM8XSW
If you want a modernish device that won't update, Voyage is probably your best bet. You can get a used one for around $50 on Unclaimed Baggage. However, I agree with j.p.s. that you should probably just get a new model and block OTA updates using the renameotabin extension.