Bebop,
One that I know works is Toshiba MK6411MAT .
I had one in mine, and I know of several others who purchased the same drive. It was a little over $100.!! 6.4 gig.
You can find them on the internet. It as been some time ago, but I found several on Pricewatch.com.
Also, Vince Cochran (you can find him on PSR Styles or Songs group) specializes in selling HDs for the PSRs. His come fully loaded with tons of styles and songs.
DonM