Cheapest way to repair the manual mode switch circuit (Code P1815) on a 2005 Infiniti G35 sedan?

Question

Whenever I'm in Manual mode, the gears will sometimes go down without any input on my part.  At times, it won't shift up or down when I do make an input.  Other times, it works just fine.

I recently got diagnosed with a P1815 Manual Mode Switch Circuit  code (no signal).  I see from a previous question with a similar problem that the answer was to replace the shift assembly.

If this is the case, can you please recommend the best (cheapest) way to get a new shift assembly?  I really don't want to go to the Infiniti dealership to get this repaired.

Thanks,
Al Corchuelo