Sounds like someone at the dealer is confused and/or doesn't know what they're talking about. I had BOTH banks, 4 cats total as front and rear cats are on one section of pipe, replaced on my SRT couple just over a month ago and the dealer had no issues getting them or installing them. They even told me at the time that they were in-stock in their warehouse.

The part numbers off of my invoice from the warranty replacement from the dealer are:

5135432-AA Converter 14070004
5135433-AA Converter 14070004

Hopefully you're getting them replaced under warranty as I was told the price was $2,250.00 each!

Not sure if I helped at all as it looks like you already had the part number correct, but hopefully you can work it out. Guess you can always try another dealer.