Answer from aFe :
3 ½ x 3 3/4 ID and 3 Length
I have found a couple place where to order couplers, thing is, it has to be soft enough to become oval. From experience, good couplers are made to be stiff and not deform.
I face the same issue couple years ago fitting a 65mm TB to a 2.5in coldpipe on my SRT-4. The solution was actually to buy a cheaper (read lower quality) non-reinforced reducer.