I can't remember the site off hand, I'm sure someone else will chime in and give it to you for the rubber gasket, they are good. But I also had a stock cover that leaked a lot. What you need to do is get a piece of 1/4" steel about 2" wide and just long enough to fit inside of the V/C where the gasket mates and check to see if the cover is warped. I am guess that it is. If you put the peice of steel in a vice and with the skinny side up against the mating surface you can gently hammer the valve cover straight again. A good flat surface, like the table on a table saw is helpful to place the cover on to see if it is twisted.
Doing this will make it so even a cork gasket will seal. Also, don't use any sealant as it will make the gasket squeeze out causing a leak.
If you get the rubber gasket you will need to remove your V/C studs from the head and install the new ones that come with the gasket. Just double nut the stud and remove the stud. Clean the hole, use lock-tite and install the new studs the same way.