Several aftermarket companies that make rocker covers make ones that are taller to accomodate exotic valve trains and radical high lift cams. However, if you are running a somewhat high lift cam, say under .550" lift and dont have stud girdles etc on your rocker arms, and you are concerned about rocker arm clearance with the rocker cover, the easiest way to take care of that is to use neoprene rocker cover gaskets that are sandwiched with a strip of sheetmetal inside them, and double gasket your cover.
Put a smear of Silicon on the gaskets before you put them together. This will asssist in preventing any possible leaks between the two gaskets. Its much less expensive and does the job just as well.
The after market tall rocker covers are anywhere from 1/4" to 1" taller than the stock units.