You will find D II's and D III's in every league, even the Cape, although D III's there are infrequent. Baseball talent seems to be scattering among the summer leagues more than it used to; I can only speak for our league, the Valley League, but 4 years ago, there were only 5 D III players in the whole league. Last summer there were over 20. Our fourth best hitter average-wise was a D III All American who hit .313, had several outfield assists, and was one of the better outfielders in the league.
Some general guidelines: (1) Freshmen position players from lesser known schools are the hardest to place-they have no college track record, and will play baseball more before June than they ever have in their life, so they are considered risky. If a freshman has a good spring, that performance might open the door to a spot opening in the Spring, and slots do come available as mentioned often on here. (2) Pitching is always needed. A freshman pitcher with a big arm can land a spot, but a freshman RHP throwing 86 is competing against 1,000 other guys with the same appearance, so without a track record, they can be hard to place. (3) A Coach's recommendation makes it much easier to land a spot. (4) If all else fails, there are a lot of leagues where you pay to play and while it's not perhaps what you had in mind, it's great experience and a good summer can boost your stock for finding a league the following summer.