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The Bun Bun Brigade
And then there's always the homemade option. This particular pen cost around $35. I(Robin) will gladly tell you how to make it. My email is on the contact page. We are highly motivated to help you successfully and happily house your bunny, and we really mean it! Please contact us with any questions.
Puppy exercise pens (x-pen)
These are great. If your floor is wood or tile or linoleum, a few non-slip rugs (traction for fuzzy feet so bunny can cavort) a cardboard box with a bunny sized hole in the side, and a litter box within the confines of a spacious puppy pen can easily make a happy bunny. Make sure it's tall enough-- 2ft for tiny or lazy bunnies, 3ft for everyone else. This pen is $35 at 2ft tall, $55 at 3ft tall, brand new. If your floor is carpet, office supply stores have plastic mats to put under office chairs in carpeted rooms or you can use the colored block mats in the toy department..
Dog crates (kennels)
A large dog kennel with 2 doors can serve well as a bunny home. And because it's made tall enough for a large dog, it leaves vertical room for "furniture", meaning a shelf to jump up on. Jumping is fun. Sitting up high is fun. Furniture is fun. A 42" long 2 door dog kennel can be purchased for less than $70. A used one on Craig's list could even be bigger and cheaper.
Hagen Living World x-large
This cage has a small front door that slides sideways, completely out of the way so that if you leave it open for her she doesn't have to step on it to get out. The top has two big doors to open up completely so you have access to the entire cage from above. Interior floor space is approximately 2ftx4ft, and height is about 2ft. This cage can be had for less than $100. If you're tempted by a smaller cage and it costs more, do not buy that smaller cage!
Third rule: try to avoid the hanging slide out tray. Really, it does seem like a good idea until you try to slide the tray out and you realize the whole tray is hanging precariously by its narrow plastic edges, sliding onto equally narrow rails. Picture your precious bunny hopping to one side and knocking the tray loose. Shock, fear, possible injury, certain mess... Unless someone gives you the cage for free... Better to get a cage that has a wire top that sits on and clips to the tray.
For those like me who like to start with firm numbers, let's talk minimum dimensions. Your bunny should be able to lay down comfortably in any direction, so the cage should be at least as deep as her body is long when she lays down flat. Your bunny also needs to be able to move around. Her digestive system works best when she moves around. So the cage needs to be long enough to take a couple full steps. With a tiny bunny, such as a Netherland dwarf, this means minimum 18"x36". Larger bunnies need larger cages, more than one bunny needs more space, and there is a direct correlation between available living space and bunny happiness, so second rule: give your bunny as much room as you possibly can.
First rule: no rabbit should ever have to live on a wire floor. It hurts their feet. If you start thinking life would be easier if the poops just fell through a grate into a slide out tray, take a moment to imagine spending all your time barefoot on a chain link fence. It would hurt. It would damage your feet. Even if you pounded it flat and coated it in vinyl you would be miserable. So no wire floors.
Ideally, you will bunny-proof your home or at least one room and the rabbit will run and jump and play and sprawl all day. Maybe Bunny runs free part of the day and stays in her "room" for the other part. Maybe Bunny is too young to spay/neuter and litter training is therefore unreliable right now. Maybe you have a dog who is sweet to Bunny, and they play together. You should still have the bunny safe in a cage when you're not around to watch them. Accidents happen, and tragedy can't be undone. For any of a number of reasons, most likely you will need some kind of cage or enclosure for the bunny.