Looking for surrogate mum/Gran

First I suggest you carry on with the boys paternal grandparents particularly as you yourself wrote that your husband's family was your family. Avoid altering any more relationships than absolutely necessary. Their father's parents are already bonded with your boys and also no doubt I expect knew your mother so they are very well placed to help you and their grandsons through this stressful period and quite likely want to. And would be upset if denied what they belief will continue. Please remember if you stop your husbands parents seeing their grandchildren their upset may not be primarily because of losing their grandsons but because of their grandsons own pain. Being put in a position where one can only watch loved ones pain because one has so little access to comfort those loved ones is and I know because I am going through it myself now again for different reasons; very, very painful and no one should ever be put through that by anyone.
Your boys have already suffered at least two different types of pain in losing their dad from home and their grandmother from their lives. And they will also be feeling the pain that they see you experience. Don't feel it necessary to always hide your pain from them of your losses. If you choose age appropriate words they can cope well and grow in strength as we meant to. Please don't make any more separations in your family if at all possible as that will add further pain for all concerned which then invariably escalates and all the relationships within your family can become acrimonious, bitter, contentious and destructive. Try not to allow that to begin because there will rapidly be no bounds and you all will be destroyed and time in which to recover lost ground can run out. I know through experience. Keep up as much direct communication with each other as possible otherwise what is said can get distorted and well intentioned people in families then so easily end up bitter. Before you know those boys will have left the nest. They are now not even as young, vulnerable and impressionable psychologically as you may fear. At their age their characters are already formed. They should be able to cope relatively easily most of the time. I have seen children much younger go through much worse and become wonderful and successful adults. Keep the number of changes in your and their lives to a minimum for a good while.
Because you may want to bring in a new partner and introducing a new potential father figure particularly into boys lives needs very careful handling and one or both of your sons may be resentful for possibly a very long time of him. Take care of them and yourself. Good luck.

Well I hope,that somehow,safely,you find what you are looking for,what about a befriending society or similar ?

I'm not sure of the reasons, trust they are good ones though.
So 2boysmum might I suggest other ways to investigate what you need.
Years ago there was an association that put like minded families together with surrogate grandparents. Maybe it is still around.
Maybe start a group yourself. All the relevant background checks needed obv.
What about taking part in the Bisto spare chair scheme. That possibly would be a start.

Think this one has been done to death. There are lots of reasons not to do this.