I bought it online, but it is now backordered. So no return and swap. I don't believe the place I ordered it from would be willing to send me just the missing sprue, because of a few different interactions I had with them.
I also checked eBay and didn't find anything.
Thanks for the suggestions though.
I did hear back from HobbyBoss and even though the kit was missing the sprue (sealed box btw) and had a duplicate of another sprue, they want me to buy the missing one. It's not a huge dollar amount, but I'll think twice about buying another kit of theirs since it was their mistake.