You are using the R3 SDK if you are searching for that file name, yes? I have written a step by step guide to installing the R3 files, including the one you seek, in a Unity project. You may find this helpful.
I do not have R3 installed on my own PC right now. But looking at the illustrations in my guide, it looks like the file you seek would be in:
RSSDK > core > plugins
The folder structure suggests that there is only one version of Intel.RealSense.Unity.dll, the one you found. Whereas the other DLL file, libpxccore.dll, does have separate x86 and x64 versions.