Choosing Optical Viewfinders for Legacy Lenses

I am currently using Voigtlander 15mm and 25mm lenses with my Sony NEX 5n. I am presuming that with the NEX's APS-C sensor, the 35mm focal length equivalent of these two lenses is 24mm and 40mm respectively. Both are fairly wide angle. permitting significantly deep depth of field setting approximations. As such, not requiring critical LCD focusing for each shot, I believe the use of hot-shoe mounted optical viewfinder(s) would be appropriate. Two questions: (1) Is my assumption in this regard correct? (2) If so, any suggestions on where I might find appropriate optical viewfinders for the effective focal lengths of these two lenses.