Rubber gaskets seem to be discontinued according to Lordco, plus they are not recommended if you have the plastic valve cover still. (but I don't have that, so we're all good!
)
There's a bunch of different methods to seal the valve cover and most use a lot of RTV. I could only imagine how much nicer it would be if there was an aftermarket silicone compression gasket.
My Fel-Pro gasket is the VS 50244 and it's about $40. It's different from the one you suggested in that one side of the cover only uses three bolts and less ribbing.