Well, the only "official" solution is the keyboard/controller combo made by ASCII sold in the japanese market. You can import one, but they typically cost around $70 plus shipping.
Then there's the not-so-official, not-so-good Datel keyboard. It is produced in the UK, and is also only available by ording it online from their website. This keyboard reportedly has several problems including "sticky" keys, laggy response, and mis-mapped keys. It sells for $20 plus shipping.
There are several keyboard adapters on the market, and this is where I think you'll find your best bet. (Ok, maybe I'm a little biased here. ;p)
The one I developed is notoriously hard to find simply because I don't feel like spending all of my free time assembling the things.

But I always have and always will be willing to provide them to AT members. Check out
this thread for details on the latest revision. $25 gets you one assembled, tested, and shipped anywhere in the US. You have to provide your own PC keyboard to use with it.
There are at least one or two other adapters available from major maunfacturers for around $20 plus shipping. I don't have any experience with them, though. There is also another independant-made adapter available from
www.indiglo.net