I'm so sorry for your loss.
First off, are you planning more rats of your own?
They benefit greatly from company of their own kind, we as humans can't replace that.
If you plan for more rats then a rescue or good breeder is the way forward, but breeders may have waiting lists.
There are a couple of options you could pursue if you've decided to take a break from rats:
You could contact local breeders or rescues to see if they have any rats you could foster from them to keep Alder company, & then return them once he has passed.
Or you can rehome him via
https://www.facebook.com/groups/RRNUK
Thank you for your reply.
It's a very good group run by a lot of people involved in rescue, but obviously you will need to do your own assessment of whether a new home is the right one for him.
It's also a good place to look to find him some company of a similar age.