After probably buying $30 worth of materials in many, many failed attempts to make a custom serial cable to connect to my USB Serial adapter, I finally got a lucky bid on a Tripplite (Keyspan) 28XG dual serial port for $30 with free shipping.

This one is supposed to be 10.6 compatible (they have drivers for 64-bit Snow Leopard.)

So I should be able to use it with my normal Mac serial cable without any modifications.

Only problem is the guy's in China, but he did have 2300+ positive. Guess I'll find out.