There are a few out there. If you're interested in mounting some kind of target optics (magnification beyond 4x) then you might want to look into the KAC G36 sight rails, short and long, as these will provide a stable base for any optics that you'd choose. They also offer a flip up rear sight and front sight if you ever need to disconnect the primary optics for aiming. They are pricey, however, so be prepared to pay upwards of $300USD when you can find them.
Most of us seem to have chosen the HK Factory Carry Handle optics if only to retain the G36 appearance from the outside. These factory optics have a very narrow Field of View, a small exit eye aperature, and are very specific to focal distance from the eye piece. In spite of that, I love my 3x carry handle optic if nothing more than it's ranging, it's designed specifically for the G36 use, and it completes the lightweight package with minimal 'add-ons'. Plus, that big carry handle makes it very easy to carry the rifle around.
Good luck in whatever you choose, and HEY, welcome to HKPRO!