Wolverine Supplies or any HK dealer may be able to find one for you, BUT for what it's worth the carry handle optics are garbage. (especially for what you pay for them)
I've seen and used the 1.5x and the 3x/red-dot combo and they are barely usable as optics. Getting a nice 1-4 rifle scope or an Aimpoint (or clone) and a flip-to-side magnifier would both be far superior options.
If you are 100% dead-set on a carry handle optic, keep your eye on the EE. I don't think most people keep them once they get some use out of them, I've seen most guys get aluminum picatinny rails and then put a decent optic on top of that!
YMMV.