Hi,
the MK4007GAL will fit in the H320 without modification (i have replaced the drive in my h320 with one of those), but as you said, they are hard to find and expensive.
The MK4009GAL has a different connector (ZIF), so you'd have to use an adapter. I don't know if such adapters are available somewhere. If you are not afraid of soldering you could build one yourself, see for example here:
http://www.stolltech.ch/iriver.pdf