Where to Find Theodor Kaluza's Paper Sent to Albert Einstein?

I assume you are referring to Kaluza's 1921 paper "Zum Unitätsproblem der Physik" which Einstein refereed and finally recommended for publication. I'm not aware of an online version of the paper, at least not in english. An english translation of that paper, and letters to Kaluza from Einstein relating to that paper can be found here: "Unified Field Theories of More than 4 Dimensions" (Singapore: World Scientific, 1983), 427-33; 447-57

Dear Terry Giblin,

Thank you for your inquiry about Theodor Franz Eduard Kaluza's paper sent to Albert Einstein. Kaluza's paper, titled "Zum Unitätsproblem in der Physik" (On the Unity Problem in Physics), was published in 1921 in the journal Sitzungsberichte der Preussischen Akademie der Wissenschaften (Proceedings of the Prussian Academy of Sciences). A copy of this paper can be found in most university libraries or can be accessed online through academic databases.

If you do not have access to a university library, you can also find a copy of the paper on websites such as JSTOR or Google Scholar. Additionally, you can purchase a physical or digital copy of the paper from online bookstores such as Amazon or Barnes and Noble.

I hope this information helps. Best of luck in your research.