Here's a link that may help in your search. It has the bed dimensions for the S10 (pages 31-40). To be safe, you may want to verify your bed dimensions against the charts. Where have you looked? Other members may know of a source you haven't found. Most manufacturers list the dimensions of their cross bed tool boxes, so you may be able to find one that fits, even though it may not be 'listed' for a 1990 S10. And, per GM Parts Giant, the older S10 short bed/ box (# 15617606) was used on model years 1984-1993, so that may help in your search.

Everything about S-series (specs) NO 56K
Credit goes to J_SIN for finding this. I figured i might as well share this to anyone who wants this. Mods, please put this in the correct section, its S10/sonoma/blazer related as well as the bagged/body dropped guys might find this useful when building a frame or need measurements. First...
And if/ when you do find one, please post it for others to reference. Good luck.