Over half the rabbits in the UK are housed alone, despite that being known as a high source of stress for them. Rabbits need the companionship of other rabbits. It helps to relieve boredom, provides constant social interaction and companionship and allows mutual grooming. Not all rabbits will get along but with careful introductions, you should be able to find a suitable match for your existing rabbit.
It is best if companion rabbits are of a similar age and size to the existing rabbits. Other combinations can work but there is always the risk of a smaller rabbit being bullied and injured by one much bigger. Both rabbits should be neutered as this reduces aggression by both sexes and generally makes introductions easier, as well as eliminating the risk of any unwanted pregnancy (rabbits do breed like, well, rabbits!).
Generally, a neutered male-female pairing is best so you will want to look for the opposite sex to your existing rabbit. If at all possible, please consider rescuing a rabbit. There are many rabbits given up to rescue centres each year, and many of these will be in need of a companion. Lots of centres will provide a ‘bunny bonding’ service to help find the right match for your bunny. The Rabbit Rehome website is a good place to start.
If you do not have an existing rabbit and are looking to get one, please get two together. If growing up together, two females, neutered before sexual maturity, or a male and female pair, also neutered before they reach maturity, is best. Two males may well fight.