Can I get these with bifocals - either generic levels or by prescription?
If not, what is the best line of sunglasses (polarized, scratch resistant) to seek for helping an old man see small flies on a stream?

Bigs, I would recommend paying a visit to your local optometrist. I've done approximately an 18 month stint in optometry, and the best thing I can tell you is to visit a professional. Call around to see which offices carry Costa Del Mar frames, or stop in and take a look at the frame selection they have at hand. One problem I can tell you when putting prescription lenses in sunglasses is that most "sporty" sunglasses frames, such as these or nike, have some degree of wrap. This is what gives them a sportier look and what keeps them on your face. A lens blank (what the lens is cut from to go into the frames) is centered and has a flat base. This leads to a prescription being off center and warped inside the frame due to the wrap. Essentially, your prescribed lens no longer lets you see clearly, because of the way your eyes process the light. There are certain ways that this can be compensated for, but generally its easier to do with progressive lenses than with bi-focals. I would look into this if I were you. If you choose to take this route, you'd be right in choosing glass lenses. They are heavier than plastic, but they are more scratch resistant and they show better clarity. And nothing beats polarized lenses to eliminate glare. You're looking at an expensive proposition, due to the RX lenses, but it is entirely worth it. A solid pair of glasses designed for outdoor use will last you a lifetime and come in handy in nearly every situation you encounter.