where can i get bayo lug for 870?

In so far as I know, no one makes one.

The original 870 magazine extension was part of a military development program, and part of that was a bayonet mount.

On the factory magazine extension you'll note the odd reduced tip that projects past the muzzle.  This is the mount for the bayonet ring.
The magazine tube has a groove that the screw for the barrel clamp fits in.  
This clamp had a bayonet lug attached to it, and the groove was to position the clamp and lug in the right position for the bayonet to lock in place.

The barrel clamp proved not to be sturdy enough for heavy use, so the military designed and built the heavy barrel band with a bayonet mount as used on the Army Mark One 870 shotguns.

What I have done for customers years ago, was to get or make a bayonet lug and weld it to the bottom of the Remington factory barrel clamp.

You can buy a cheap M-16 front sight and cut the bayonet lug off, then have a good welder heli-arc or Tig weld it to a factory barrel clamp.

You will also need a factory extension.

This will work, BUT...keep in mind that if you really use it, you run the risk of bending or breaking the magazine extension off the gun.
This usually damages the gun's magazine tube, and that requires an expensive trip back to the factory to have a new magazine tube brazed into the receiver.

OPTION NUMBER TWO:
Get a full WWI type barrel shroud and have a bayonet mount welded to it.
You'll probably have to modify a bayonet mounting ring to fit over the muzzle of the gun.

You can buy steel barrel shrouds for the 870, but these tend to fit loosely and slip forward under recoil.
If you can get one to stay put, the bayonet mount would be stronger than the magazine extension mount system.

In other words, how badly do you want a bayonet, and will it be a really usable bayonet or just something that looks cool?