Eagle and Ramline both made transparent plastic mags, but neither are worth buying.  The plastic is very brittle, and because the AR was designed for metal magazines, the magwell doesn't allow mags to be thick enough for transparent plastic mags to be strong enough.

The Steyr, HK, and SIG rifles were all designed for plastic mags from the beginning, and are set up to accomidate the much thicker mags.  Plus, the plastic used for those mags is much stronger (and more expensive) than the stuff used by Eagle and Ramline.  Remember that when those mags were being made in the late 80s and early 90s, they were competing with $10 NIW USGI mags, so any company that made aftermarket mags HAD to come in below that price.

I'm sure you'll find some of these mags if you look around the EE or the gun sale/auction boards, but know that they'll break after a few uses and they aren't repairable.

-Troy

Well, you could look at a loaded mag and determine that it was fully loaded, I guess, but the AR mag well is so deep that you could have anywhere between 15 and 0 rounds remaining and you couldn't visually tell the difference.

And it is very easy to drill a small hole in the mag body to be able to verify that the mag is fully loaded.  AK mags have had this for 55 years already.

-Troy

You may have seen an AR converted to use Simunitions.  Simunitions guns shoot something like a 9mm cartridge with a paint capsule for a bullet, and the mags are clear plastic and designed specifically for the kit.  Of course, Simunitions products are sold only to government/police agencies and to those who train such.

-Troy