I just went through the same and I'm afraid I have no ideas for you. We tried for 6 months, and ended up surrendering her to the local version of the Humane Society with crossed fingers. I just wanted you to know it's not your fault and that there is at least one sympathetic person out there.
People just want kittens :(
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25yo FTM to a Wiggle Panda , student teacher , newlywed
Depending on their temperament (are they friendly, very loving, non-scratchers (or, declawed), indoor-only), you might contact nearby assisted living residences (ALF)/communities. Don't know how things work, in your state, as far as health department + ALF animals.
Years ago, when we were looking at ALF residences for my Mom, several had cats "on staff." Mom was no lover of cats and so this eliminated a few possible places! But, for those folks that liked cats, they seemed very happy having a cat curled-up, asleep, in their laps.
Just a possible idea.
If your cats are outdoor or not good with strangers, than I can't think of any other ideas. Please do not drop them off, in the country, thinking that all rural folks want cats to keep down rodents and will take them in. This is a continuous problem in our area.