Need more info on what you have
Stock heads & stock valve covers, you're stuck with the cork & rubber usually 3/16" a minimum thickness.
or
Aftermarket heads & Cast or Fabricated valve covers, I can help with this as I offer a .050" thick gasket.
If you have Stock heads with Cast or Fabricated valve covers and feel ambitious you can remove the heads and have the valve cover gasket rail machined true & flat so a thinner gasket will seal. In fact this will help ANY gasket seal better for you.
Just some food for thought
wish I would have machined mine while they were off. It WILL be done next time.
Greg
Oh forgot about the rubber gaskets. I think Fel Pro or somebody makes an 1/8" rubber gasket but don't know how well they seal