Most of the cheap polycarb lamps are Chinese made. The polycarbonate used is soft, and very prone to scratching; road debris ends up sticking to the plastic, much more than glass, so the lamps can haze quite quickly.
Anything that fits a classic Mini or Landrover will fit a NA Miata. I believe new LR Defenders are now fitted by the factory with Wipac freeforms. The Mini or LR part will also have a sidelight fitting, but you can leave this unwired. So check with US LR and Mini aftermarket specialists.