I was going to say The Bivouac, but when I went looking for traces of it, it turned out to be a customs site, not a dio site: http://www.joecustoms.com/bivarchive/ . Still, I vividly remember the home page of The Bivouac showing a picture of some Joes in the Bivouac playset placed in natural surroundings, so that was definitely a dio. So The Bivouac still counts, imho .

Apart from that one, I only remember the diostory-sites of those days; Iconoclasts and GIJoe Warfare and the likes. I also remember a website that had some scans of old catalogue dios (another website than my own, that is), but I don't remember which site that was.