hi all    ive removed all abs hardware from my k1100 1999  and im looking for a rear disc rotor and pads non abs , does anyone know what rotor fits and where to buy any help would be appreciated

I'd quite like to know the answer to this too. My '91 K100RS 16V ABS needs a new rear disc. The ABS has been removed.

It looks to me that the wheel (3-spoke), final drive and brake caliper are the same part nos on ABS and non-ABS models (K10016V and K1100). And the only difference in the brake disc is the ABS ring or, more accurately, the flange and securing screw holes for the ABS ring. Motorworks sell an EBC disc for the ABS bike that requires you take the ABS ring off the old disc and fit it to the new. And they also do an EBC disc for the non-ABS bike. Are they the same? Or the same minus a couple of screw holes?
And Brembo do a rear disc for the non-ABS bike which is a bit cheaper.

Are they all, ABS aside, interchangeable? I.e. same offset from flange to rotor? Same flange thickness?

I have a non ABS bike and I managed to pick up a new set of fully floating ABS rotors really cheap. The only difference is the flange for the ABS ring. I have measured them up against a non ABS fixed disc set. If and when I get around to installing them I am undecided whether to leave the flange on or machine it off.  
Regards Martin.

Never noticed the difference, wouln't worry about that.

Rear brake discs, whether ABS on non, per the requirements of the bike, are the same diametre, thickness, offset and function.
I've got the same part number EBC 'floater' on Big Block (originally ABS) and red '87 RS Down Under.