Ah, yes. Good ol' coin roll hunting/silver surfing.
Kennedy half dollars were 90% silver in 1964 but between 1965 and 1970 they were 40% silver. I find a 40% one every now and then but the 90% ones are not out there. After 1970 they have no silver in them.
90% silver Kennedy halves are out there, just maybe not in your reserve banking area. How many boxes of halves have you searched through? I have a few rolls worth of '64 Kennedy halves from my CRH escapades. Also found a couple Walking Liberty half dollars from the early 1900s. I did go through $10k in halves the first 2 weeks I started though. Talked a local Bank of America into ordering them for me in return for signing up for an account with them.
What I have yet to come across are any Benjamin Franklin halves or any 1970 Kennedy halves. I can pull a stack of '64s so high that they'll topple over before I can find a '70. I've only seen a '70 in a coin store.
Some of the '64s I've found have been the only thing that was silver in the entire box but were missed by people who edge check exclusively because they had a band of dirt around the edge that people mistook for copper and threw them back in their dump pile thinking they were clad.
So far, I've pulled over $1000 of melt value in silver. Not bad for a part-time hobby.
The real challenge is getting banks to order you halves before they get tired of it and cut you off.