It is better to get the gun complete with the tool (which does more than just fit the barrel nut) & gauge, but if you can get one without for the right price there's a company called "EWK arms" that makes & sell the wrenches. Be aware that there have been several styles of barrel nut & you'll need the right type of wrench that fits your muzzle nut design.
The gauge can easily be replaced with a standard automotive feeler gauge set, just use the .005" gauge.
Allan Quartermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”