Please be gentle, this is tough for everyone involved. My Dfil died at the beginning of the year, leaving dmil and 3 boxers.
Dmil has dementia and we need to bring her closer to us, I'd have her to live with us in a heartbeat, but we have two greyhounds and we can't cope with 5 big dogs. The boxers are a mother at 11 and two of her children at 9.
There are lots of Boxer rescue services, I just googled Boxer Dog Rescue and there are different organisations around the country. I suggest you follow some on facebook if you are on FB and check if they seem reputable - home checks, questionnaires for potential adopters etc.
I know it's a huge ask, and my DH would probably say no too, but they'd probably be really easy dogs at that age, mostly sleepy and laid back and not needing much exercise. Plus, I find the dogs massively help keep my MIL with dementia calm.