What can i replace my keyboard with to be compatible with the latest possible updated system for my 2009 iMac?

If you have been using a wired or wireless Apple keyboard, there are plenty of good used ones available on eBay at reasonable prices. Check the description & photos carefully, scope out the seller - check their feedback reputation. I also recommend only purchasing from sellers who have a 30 day return.

OWC also sells Apple and Apple-compatible keyboards. They are new and have warranties but they are also more costly.

There is no relationship between the kind of keyboard you use and the version of macOS on your iMac.

if it were me, I would do my shopping for a keyboard at OWC’s website. they carry Apple Original keyboards as well as third party keyboards. they have them in both wired and wireless models.

but you could get a keyboard from eBay or amazon or something. just about any keyboard can be used with your Mac.

and finally, changing your keyboard will not make a Mac eligible for macOS upgrades if you are already running the most current macOS your Mac is compatible with.

I use Apple, Macally and Santech keyboards and all work fine with my iMac but has no impact on the OS.

Getting an Apple keyboard for an older Mac is a bit like walking through a minefield.

I don't think any of Apple's current keyboards are compatible with El Capitan, so if your Mac is an early 2009 one, a new Apple keyboard would not be an option for you.

The Magic Keyboard with Numeric Keypad for Mac models - Apple requires macOS 10.12.4 (Sierra) or later – which might be doable if your 2009 iMac is a Late 2009 machine. All of the others require something later than any 2009 Mac can run.

The latest possible macOS version for an Early on Mid 2009 iMac is 10.11 El Capitan. A Late 2009 iMc ant run mot higher than 10.13 High Sierra. AS ther are not six newer OS version since 10.14, it is unlikely your have any OS useful OS upgrades available to you