Looking for a window sill... please advise.

Hello home builders,
We ordered large gray interior windows, but they mistakenly arrived in white, which we were actually very happy about. A blessing in disguise.
The problem now is that the window installer used a gray strip as a finishing extension.
He also installed it terribly, so I am now completely dissatisfied.
My question is:
I am now looking for a strip about 10-11 cm wide and approximately 30 meters in length in white.
The plasterer is not finished yet and would nicely plaster it on the side.
What options do I have? What strips would you recommend to me now? I would really appreciate your help.

The problem now, however, is that the window installer used a gray strip as the finishing extension. He also installed it terribly, so I am now completely dissatisfied.

I don't know what a terribly installed finishing extension is, but the window's appearance should match the strips. If the installer messed up something with the color, it's his responsibility to fix it properly. So, in your place, I would contact him to have it done correctly, meaning corrected. Because finishes should probably blend in inconspicuously there.

I see a picture ... but don't see anything relevant that would reveal your problem? More meaningful pictures would be nice

I see a picture ... but don't see anything relevant that would reveal your problem? More meaningful pictures would be nice

I see a Dalli-Klick picture that presumably shows the left jamb of a patio door, which (for whatever reason) is covered with a dark film. The effect is roughly the same as in the Dr. Zhivago sketch by Didi Hallervorden – in any case, not remotely enough that I, as a former window manufacturer, could even begin to guess what might help.