Remember Their Needs.
A lot depends on how many rats that you have and how big your rats are. However, here are the things that you should remember when looking for a rat cage.
Each rat that you have needs a fair amount of space. It is acceptable to offer them about 2 cubic feet of space (per rat). There are sites that make it easy for you to figure out how many rats will fit in a cage including The Ratty Corner Cage Calculator.
Large rats do okay with large bar spacing, but most rats aren't that big. You really do need a bar spacing under 1". Often this is 3/8" to 5/8".
The size of your cage isn't as important as what you put in it. You want to load it down with as many things to do and as many different ways to fill up the cage as possible. Check out the article Rat Cages: Accessorizing Super Cheap.
You have a couple of different options for shopping. Often it is best to stop at the pet store first and then you should get online if you can't find anything better. Online you will find more resources, but there are many advantages to seeing a cage in person.
A Note About Ferret Cages, Guinea Pig Cages, and Other Cages.
Sometimes one has to choose a ferret cage, a guinea pig cage, or some other cage. However, it is best to do this with care. You don't want really large bar spacing or you may find a rat that loves to leave home to go "visiting". On the other hand you should remember that hamster, gerbil, and other small critter cages are likely going to be too small. Additionally cool tunnels, wheels, and secret rooms are going to be too small too. If you have a baby rat you should remember that your little baby is going to get much larger.
While as a rule of thumb these are not great options. However, there are times when these are the best choices out there. If you have large rats and feel confident that you won't be getting any small ones any time in the future then the bar spacing isn't as big of a deal. Some rabbit cages have smaller bar spacing and every now and again you can find a really large hamster cage that will work if you accessorize well and you don't have more than a pair of rats.
The Real Mansion.
There are some cages that claim to be a rat mansion. However, one of the coolest cages you can get is the Critter Nation Cage. Here you will find a 2 by 3 foot cage that is either one floor with a shelf or 2 floors with 2 shelves. Both versions come with the ramps needed to easily get from one location to another. This cage is designed from the Ferret Nation (same cage, but with different bar spacing). It has the prefect bar spacing, so you won't have to worry about your rat getting out. This monster cage will let you load it full of toys and give your rats the mansion they always wished they had. Well...maybe not since rats will love their cage whether it is 2 by 3 or much smaller than that.
There aren't many "rat" specific cages out there. They aren't in as high demand as guinea pig cages, rabbit cages, or ferret cages. However, Martin's Cages are designed for your little ratty and in mind. They come in a huge variety of shapes and sizes. You can pick out the cage that will meet your needs and your budget. They aren't as huge as the Critter Nation Cage, but they come in a wider range of sizes and prices.
Martin's cages are galvanized, safe, and come in the size that you need. They also have floors, ramps, and shelves to make the most of the space that you get. Most don't have enough room for a lot of rats, but at the same time they all work to make the most of the space they do offer you. Many people love Martin's cages and they are some of the best "rat" specific cages available.
Another idea that many people don't think of, but that has the potential to work great are bird cages. You can find many bird cages in a wide range of sizes. You want to make sure that the bar spacing works for your rat. You also want to find a cage that is easy to clean and one that has doors that will open wide enough for easy access to your rats. Make sure that the cage will be easy to put shelves in and to hang other accessories.
Taking plenty of time to find the perfect cage is the best option. Most of the cages that are available are really small and you don't want to bring home a tiny cage for your rats. The more rats you have the more space that they need. If you can afford it the Critter Nation Cage is by far the best in offering a huge amount of space, providing the right bar spacing, and being easy to clean. It is also a cage that is extremely durable. It won't be chewed threw and it will stand the test of time for years (and years) to come. If you have several rats, can afford it, and can find a location for it then it is definitely worth going for!