To the best of my knowledge, MSG isn't actually found in most sources of umami, but it's an excellent source of that flavour.

Well-known umami-tasting foods include red meat, cheese and other fermented products, and tomatoes. None of these contain MSG directly (tomatoes get their umami from glutamic acid, according to umamiinfo.com). However, MSG can be used to add more umami flavour, which acts as a flavour enhancer.