Salvage yard if you can find a ti or use the rear calipers and carriers from a e34 325I. Note that this is a rear brake upgrade and will change your brake bias. Good reason to upgrade the front setup too.
For normal driving, the stock rear with good pads are just fine tho...
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I know there are at least two TI's in the bay area that I saw still had their rear brakes if we can get a forum member to pull them for you. San Jose and Richmond, CA pick and pulls.