You are asking for a FireWire to USB converter. No such thing exists. (All the combo USB/FW hubs require both connections to the computer.)
I agree that a CardBus USB 2 card is probably the best way to go. The only way to go, really! Whether the card also has FireWire or not sounds irrelevant, since it sounds like the TiBook's FireWire ports are in fine shape.
tooki