LOL at that guy's response. AmigaKit doesn't have replacement A500 keyboards. They haven't in god only knows how long. At least not listed on their website. :p
A quick ebay search for "Amiga 500 keyboard" yields dozens of results, in varying shades of yellow, unfortunately they're all in the UK at this time. You could of course buy a complete A500 that's otherwise broken and just salvage the keyboard from it, but unless you find someone dumping one on craigslist or some similar site, you'd likely pay more for a complete system (even broken) than you would just a keyboard.
There's always Amibay, couldn't hurt to post there in their "wanted" section. I've had some good successes on there in the past, but unfortunately most of the members there are in Europe, as well.
If you're able to find the folks that were selling stuff at Amiga30 in Cali, there was a guy there with a box full of A500's in various states of disrepair, going for $50 each. I don't know who that seller was or how to contact him, unfortunately.
Of course your last option would be some kind of A500-to-PS/2 keyboard adapter and try to hack that to fit inside your case... but yeah. Please don't do that. :p
Good luck with your search, they come up fairly often in the US, unless you're impatient you should be able to find a good one here in the US, in a few weeks or so!
