There's one item in the game that can be missed, but I'm not sure if it's either of the ones you're describing. I do believe that the one in the elevator can be gotten after you go through the long, curving ramp with all the enemies that teleport in out of nowhere. Use a Power Bomb on the glass tube that encases the elevator to melt it, then look for a Morph Ball tunnel in the immediate area. Roll through and you should find a Missile Expansion.
I wish I could be more help, but I don't know the game very well myself, and I've no particular inclination to play it again, since I'd have to put up with the Ridley scene.