Pep Boys is clearancing out some of their old stock parts & selling them on Amazon. The rubber seal is included.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
They are selling the Stant 180 Degree thermostat for the LS5 for $15.29ea. I "Believe" that this is a very good price, I have seen them listed for sale for over $40 at some sites.
I bought 2 and they still have plenty of stock.