Hello,
and welcome to the forum, camrex (Robert).
This is something I wrote for another member; seems the question arises every now and again (no offence meant

)
Well, my first guess would be to contact Stan Sakai himself, especially if you've got something specific in mind; As far as I understood, the best way to actually acquire the piece would be a Convention or any other of Sensei's appearances. But these mere details could be discussed with Sensei himself.
Waiting for an ebay auction might seem another option, with several disadvantages:
1.The art has traveled through several hands, so the price went up alongside. Cut out the middle man, I'd say.
2.You can't be really sure about the condition your piece arrives in (images might not show every flaw, postal service may pose a threat in itself).
3.The supply on ebay is limited to what other people are willing to part with, which is not necessarily what you'd like to have.
4.But the major disadvantage is that you miss a chance to meet Sensei, thus giving the purchased art an extra story to tell envious friends
There may be some original art around in some comic shops, but I doubt it.
Good luck finding your piece of art
Edit: Contacting Sensei should be simple through this forum; believe me, he's a really nice person. But keep in mind that he might be busy....what with drawing all these comics, original art.... oh, and traveling...not to forget his family...