Actually, more specifically, no one is making an external enclosure with a SATA II "external" connector. Those SATA II drives you pointed out have the regular old SATA I "internal" connector on them. But, you don't need to buy a SATA II drive to work with the 8300HD - any old SATA I drive should work fine (that's what I'm using).
The SATA II spec. added a special connector for "external" connections that's more robust, etc., and this is what the 8300HD has on its backside.
That's why you need the SATA I to SATA II cable.
Hope this helps.