I don't like trendy "must haves" but I must tell you, classic "must haves" are indeed MUST HAVES, and the blazer fits into this category quite nicely.
Great classic fall piece, and well priced. Try Gap (below) or the little boys section of any department store.
The eighties are trying their darnedest to come back (as we saw Marc Jacobs declare this past spring) and while shoulder pads should remain in the past (forever please!) a little eighties infusion ain't so bad. Example below.