Why did Itachi say that he needs to get someone's MS to get EMS while Sasuke only had a normal Sharingan?
Because he actually didn't want them. He was trying to relay that information to Sasuke before his death. He gave that information about EMS to Sasuke so that Sasuke could obtain EMS after his death if he wanted.

Plus Itachi was on the verge of blindness. So even if Sasuke had normal sharingan Itachi could at least have sight, that's a good excuse.