If you are serious about this, I can tell you that in August of 2012, it was reported that Jim Steranko was doing some commission work:
From Arlen Schumer and Kurt Busiek:
Jim Steranko has finally agreed to accept commissions from art collectors in the States and overseas. The subject matter should feature characters identified with the artist, such as Captain America, Nick Fury, The Shadow, Batman, etc. He generally works in two standard formats:
Character portraits rendered in ink on 7x10" stock which range from $250-750, depending on the complexity of such detail as dramatic lighting/shadows, expressions, minor background elements, etc. Blank comicbook cover portraits range from $400-750. Super-tight pencilled figures with minimal backgrounds on 11x14” Strathmore vellum stock run about $1250. Variations will be quoted individually. Scheduling depends on commercial deadlines, but generally 1-3 weeks. Postage, packing, and insurance are extra. Steranko vintage originals have recently sold for $30-100K, with recreations reselling for $30K, so the investment aspect is significant.
The artist can be contacted at supergraphics@prevue.net.