If it doesn't have to be Dinan per se, you could look at strongstrut, which is still for sale I believe:
http://www.strong-strut.com/Z4/front...20strut/Z4.htm
And arguably better (stiffer?, lighter?)
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Z4 3.0i | ESS TS2+ supercharger | Quaife ATB LSD | Brembo/BMW performance BBK front/rear | Schrick FI cams | Schmiedmann headers+cats | Powerflex/strongflex PU bushings | Vibra-technics engine mounts | H&R anti rollbars | KW V3 coilovers/KW camber plates | Sachs race engineering clutch | tons of custom sh#t