I have an AF7925 Leland Engineering pin box I'll let go real cheap. If you look at the Label on your pin box, it'll tell you the manufacturer of the chassis of your 5th wheel trailer. Other than that, your best bet is through your local RV dealer as far as availability. They're about $200. Mine is about $50.
I went back and read your signature again. You might want to check with the manufacturer before going with more than a medium extended pin box with a trailer that heavy.