eBay seller 'fluke-usa' is reliable, but I think he's out of those now. You might e-mail him and see if he thinks he'll ever get them again. I bought a back cover (and had to pay extra for the screws) for what would might seem an exorbitant price, but the value of those 189s has soared to absurdity these days. The parts ran out because industry users released a pile of those as surplus after the support ran out, and a lot of them were in really beat condition and people wanted to fix them up. If you want some with the battery door and stand missing, sharpie markings all over the case and department names carved into the front panel--but still working 100%--there are plenty to be had. For the right price...
Just check eBay every few hours for the next few months and maybe something will turn up. And be quick!
Edit: do you need the door or the screw part?