Advice - finding home for cat

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Hi all, I'm hoping you have some suggestions for me:

My FIL took in a stray cat ("Mama") back in April. She's only about a year old. Turns out she was pregnant and she quickly had three kittens - we found homes for two and we're keeping the third. 

Then FIL got sick, and was in and out of the hospital from June until he died rather unexpectedly in early October. We took Mama to our house in June and she's been there ever since, since FIL was not well enough to take her back. We isolated her in a spare room with the door shut, since we have two adult cats of our own and she doesn't get along with them. She escaped the room accidentally one day and bit one of our cats, so I think that even a slow re-intro is impossible at this point. Even if we could magically get them to be friends, four cats is just way too many for us.

We've been trying to find Mama a new home ever since FIL died. We got her spayed, vaccinated, and microchipped. She's at least safe and contained in our spare room, but she doesn't get as much attention as she got with FIL and we feel bad. We don't want to bring her to a shelter and have her sit in a cage all day, but we really can't keep her forever. 

A local rescue group has been helping me by listing Mama on their own website and on Petfinder, and they let me bring her to an adoption event last week but nobody expressed interest in her. I've posted her Petfinder profile on Facebook and asked my friends and coworkers to share it (but it was hard enough to find homes for her two cute kittens, so I think I've exhausted all my resources with friends and coworkers). I'm going to ask local seniors' centers if they can post a flyer about her. I even Googled to see if there's a prison pet program in my area but I can't find any. I also Tweeted the Mutts comic strip and Howard Stern's wife, since they are both local to me and heavily promote pet adoption, but they didn't respond to my retweet requests.

Are there any other strategies I'm not thinking of? Any suggestions would be appreciated. I'm in NJ, if that helps.

Thanks.