Where can I get a copy of the pre-release bonus pack for Nightfall?

I've looked everywhere but I can't find any vendors for the product key. Amazon has a grand total of three sellers who are selling at the exorbitant price of $185.

Tbh I'd consider you fortunate to have even found a key at this point. Remember that the keys you're looking for are now nine years old, and there has certainly been interest from others in obtaining them. A thread seems to pop up here every now and then about them. Supply and demand principles apply, so if you're not willing to bite the bullet on those Amazon listings you'll just have to keep trawling through every retailer you can find in the hope there are others (I suspect online ones would be better). You're not going to find one for $10.

Man, this is just silly. Was the Nightfall one insanely popular or something? The Factions and the EotN bonus packs are less than ten bucks on Amazon for a fresh copy.

Getting an extra character slot for below MSRP must have been appealing. I bought a half-dozen of them back in 2010 for something like $1 each before shipping.

A lot of pricing on Amazon Marketplace is done via automated means; a list price of $300 probably doesn't mean that anyone's buying it at $300.