After saving Joey & Téa, & then going to the aquarium & dueling Magnum, but before going to the museum to duel Ishizu & Kaiba, go to Kaibacorp, & Mokuba will be outside. You can duel him once. After getting your sixth locator card, before going back to the toy shop, you can still duel Esper Roba, since no-one knows you've got all six cards yet. After splitting up to look for clues, you can't duel Esper for the remainder of the game.