I think what you're looking for is called a "reducing wye" and will be identified as a "4x2.5x2.5". The first number (4) is the dimension of the outlet leading to the dust collector. The second number (2.5) is the diameter of the inlet coming from the tool and directly in line with the 4" outlet. The third number (2.5) is the diameter of the inlet that branches off to another tool. This numbering system is standard in DC terminology.
As for where to find it, I'm not sure. Check with all the online suppliers of DC duct work. If you have any recent woodworking magazine, they all have adds in them. Otherwise, Google "Dust collectors" and see what you get.
HTH, Bill Clemmons