Places can get you rubber gaskets as I think speco make em or get em made for like Repco,Bursons and such.I use a cork unit and don't have leaks because I get some Permatex black RTV sealant and smear on a even smooth layer on the bottom of the gasket and then clean (with a rag dipped in thinners) the rocker cover mounting mounting surface on the head and the rocker cover and place the gasket on the head.Then a very light smear of fresh clean oil is smeared on the top of the gasket then it's time to bolt the cover down in a centre to the ends sequence and leave it for atleast 24hrs.
Admittedly I run a big fat Aussie speed cover that doesn't leak at the worse of times but if you just prep a basic cork unit they will hold up.Or order in a $35 rubber unit and don't over tighten em.