A lot of the time if you contact those places directly (eg, kings, eibach, whoever) they can do a special order for you and make them to exactly you're specifications... but I'd assume that it wouldn't be necessary. Surely there is someone out there who makes springs for a mk4 supra that was only slightly lowered.
Also try looking on www.takakaira.co.jp or www.greenline.jp (no .co) and have a snoop through their range. At least it'll give you an idea of what's available, even if you don't weant to order it from japan.