H1B extension, I-94, immigration, USCIS application procedure

Your employer or the attorney who worked on the H-1B should have a copy. You can get by with just a copy in most all situations, since the American consulates now do not rely on the original approval notices any more.

If you need a duplicate copy you can file for one using Form I-824.

You may also be able to file for a replacement I-94 on a Form I-102. As the previous writer said, you might be OK without an I-94 and you can get a new one next time you return to the US from international travel.