Yes older bunnies are few and far between in rescues - which I guess is a good thing in one way, but it is difficult when you lose one when they're older. I lost my Harry back in July and I was looking for an older bun to bond to my 9 year old Heather. In the end I adopted a 4 year old because I just couldn't find any single older bunnies who seemed suitable. As I have the space, I did get a bigger bun though as bigger rabbits tend to have a shorter lifespan than smaller ones, so a 4 year old french lop for instance is much "older" than a 4 year old dwarf lop.
There are a few older buns around in rescue but they tend to get snapped up fairly quickly especially if they are single and in good health. I would suggest getting in contact with some rescues to see what they have, it's very possible that they just haven't got any advertised as they tend not to have time to put them all up. Other than that, there's no reason why you couldn't bond buns of different ages, I would just take care to ensure that you get one with what seems to be a compatible temperament, one that is a bit calmer and soppier than a livewire, for instance. Different temperament and age bunnies do get on but as they get older I think it's better that they have a calmer friend if possible.
Good luck x