Actually GW used to sell a boxed set for the cult army back in Rogue Trader days. The army list, as much as they were back then, was published in White Dwarf 115-120 ish (a little big of search -fu can dig it up if you want). The limos themselves where scratch built for the studio army of the above. The white dwarf was fantastic as was the beautiful artwork and amazing fluff about stealer invasions.
edit: pretty good noggin. White dwarf 114, 115, 116 had the stories, army, art work, etc. This was to coincide with the 1st edition Space Hulk game release and to bring the Genestealers into, what was modern look/fluff for that time, with 40k. Also to note, at that point they were not tyranid vanguards.
edit 2: found some amazing pics of the infamous stealer limos. Evidently Bryan brought them out for a convention and the old hammer guys were able to get some uber detailed pics.
http://forum.oldhammer.org.uk/viewtopic.php?f=6&t=379 scroll down to the bottom for the side and under looks
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