Hi Pickles,
I’m sorry to hear about your loss. Bunnies that are used to having a companion often do get depressed when their friend dies. It does sound like he could do with a friend. Is he neutered?
The best thing to do is take him to a shelter and let him ‘pick’ his own friend, by trying a few dates and seeing who he gets on with best. Both bunnies must be spayed/neutered. If that’s not possible, then think about the bunnies’ personalities when choosing.
Boy/girl combinations are generally easiest, but its the personality that really matters. Age doesn’t really matter, but a very young bunny with a very old bunny can end in the young one being bereaved early, so roughly the same age should be aimed for, although it won’t affect their bond.
Once you get your second bunny home, you can spend some time pre bonding them, before bonding them in neutral space. Sometimes grieving bunnies bond incredibly quickly, so it might be an easy process, but it could also take a couple of months.