Hi,
I have a TL-WDR3600 with OpenWrt 18.06.1. I wanted to upgrade it to the latest 18.06 release, 18.06.9... but I can't find the correct image. I can find the latest 22, but not 18.
Devices with these chipsets used to be in the ar71xx target. They migrated to ath79 starting in OpenWrt 19.07
But why do you want to use an old release such as 18.06? It has been EOL for many years, contains many security vulnerabilities, and is entirely unsupported.
Your device supports 22.03... is there a reason you don't want to (or can't) use it?
Old versions are stored in archive.openwrt.org. 18.06 can be found by going to the downloads.openwrt.org index page and following the links. But you should not use it.