Does anyone know where I can find this sort of l-screw hollow wall anchor combo?
Limo is correct, you can replace the machine bolt with an L bolt/hook that has the proper threads.
I prefer Toggler anchors for superior weight holding ability. (Limos advice would work for those as well.)
BUT,  positioning both hooks level and spaced precisely for the keyhole hangers is a challenge.
If the back of the painting will allow it then i would use  the French cleat method. If i do not construct one myself from wood then i  use the Hangman Picture Hanging system.
It may bridge two studs or at least one, and the picture can slide side to side somewhat if you need.