Source for BSF cup square coach bolts?

Does anybody on this group know of a supplier of cup square coach bolts in 3/16th BSF? I need a couple of these to repair the airconditioning fans in my car. Each fan has two 3 inch bolts, and the square behind the head seats in a square hole in the casing, which is about 3/16th, or roughly 4.5mm. The other half of the casing is then secured by a nut on the end of the bolt.
It seems impossible to get coach bolts in anything less than M6 thread, which is of course far too long.
I could possible make do with a plain hex head bolt, but would like to replace the original bolts.
Is anybody able to help me out here?
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