Can you ever say Kaddish without a Minyan?
You can say Kaddish without a Minyan if when you started Chazaras Hashatz you had a Minyan and someone left in middle you keep going and even say Kaddish!!!
Just to clarify...if you had a minyan for the silent amida and then the tenth walked out so that you are left with 9 you may say chazarat hashatz (repetition), and say kaddish shalem after uva le'tzion but you may not say chatzi kaddish after tachanun nor may you take out the torah and read @user146 is correct, I would also add arukh hashuhan who talks about when the tenth person leaves.
I would also add that the Shulkhan Arukh cites a yesh omrim that allows for all devarim she'bikedusha (things which require a minyan) to be done with 9 and a katan who is holding a chumash or 9 plus a sefer torah. Most dont use this even in a shaas hadachak but many will rely on this in the shaat hadahak situation.
rema 55:4 ואפילו על ידי חומש שבידו אין לצרפו. מיהו יש נוהגין להקל בשעת הדחק and even if the minor is holding a chumash it is fobidden to include him. However there are those who are lenient in a shaat hadahak (instance of dire need)
the rema is citing based on the RO"Sh and the Hagahos Maimoniyos and the Mordechai