EDIT: I'm leaving what I posted incase anyone else wants to reference it... can't imagine why they would though... After re-reading your post I believe you may have just meant the stock three piece spoiler... it doesn't sit a half foot over the hatch, but the end pieces extend that far forward on the fender? I believe you'll have to buy one used somewhere.
Well, the only stock Supra spoiler that sat that high is the MKIV's spoiler and, IMHO, it looks like crap on the MKIII... it's too round and looks better on riced out Civics (very popular among them). I've seen some pics of a red MKIII with the MKIV spoiler, shouldn't be hard to find, it's ummmm... popular.
Just do a google image search for Supra wing or Supra spoiler and you should find an example of the three stock wings I mentioned.
There were two aftermarket spoilers made for the MKIII specifically which are taller. One mounted like a second spoiler on the three piece spoiler which came on 89 and newer MKIIIs and the other was a full replacement three piece which looked just like the stocker but sat a good bit taller. I have no idea where youd find either though, I'm quite sure they're not made anymore. I'll see if I can find the brands for you though.
Or, if yu feel like really shelling out a ton of cash you cold always pay an import company to get you a S2 wing out of Japan:
http://s2.supra.gr.jp/products_04style.html