Yes, there is a vinyl fire pit cover available along with a glass burner cover.

Is this for additional purchase or does it come with it?

Both are additional accessories that you can purchase:

Vinyl cover: https://www.houzz.com/products/rectangular-37x27-vinyl-cover-for-prividence-fire-pit-tables-prvw-vr~23573865?lid=11988134

Grey Glass Burner Cover: https://www.houzz.com/products/glass-burner-cover-for-cf-1224-burners-grey-prvw-vr~23573959?lid=11988198

Does the stainless steel top get discolored with weather or the heat from the fire ? Is a stone top better ?

The stainless steel will not get discolored due to heat. It has been tested to withstand the heat of the burner. Outdoor GreatRoom does recommend using a vinyl cover when not in use (such as during the winter when the pit is not in use).