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The problem of such sensor are that they are mostly fabricated for OEM's.
So there is nearly no chance to get a single one.
One possibility is to get a door-opener and to reuse it's doppler-module
(most a gunn-source with demodulator). Most of them are working on
the K-Band (24GHz), but there also exist (bigger ones) at X-Band (10GHz)
Perhaps you will find on the internet something if you search with terms
like 'gunn source', 'doppler module' or 'movement sensor'