Rear sight or front sight or both? If just the rear sight and the thumb safety, go on a fun adventure and find them yourself at a gun show or where ever and install them. Just be careful to match finishes/ The search is part of the fun in owning and maintaining these gems.
A lot depends on what you really would like to have in the end.
I don't know in what part of the States you live, but perhaps you could contact Mr Green, FWIK his works look often like masterpieces, and not only on Lugers.
I second the idea of looking for the parts at gun shows. Or even flea markets. I much prefer vintage parts over repros if at all possible. At one gun show, I saw an arched checkered MSH from an unknown brand of 1911A1. It was in (of all things) what looked to be a jelly jar along with some screws and a a hammer from some kind of single action revolver. I left the show with my wallet lighter by $12 and the MSH in my pocket. The guy didn't even know what a MSH was. I almost felt guilty, but not guilty enough. Well, I couldn't just leave it there in that jar, now could I?