Buy vs. Rescue
There are a few things you should decide before you start shopping around. Decide on the colors or patterns that you like most. Also, you need to know ahead of time whether you plan on showing your Persian. If you don't plan on showing, then you probably want a "pet-quality" cat. You should decide whether or not the age and sex of the cat matters, too.
Once you get a feel for the kind of Persian you want, talk to a vet and see if he/she can recommend a Persian cattery in your area. Most likely, the vets in your area will know which catteries are best known for producing healthy, well-adjusted animals. Another option is to visit a cat show, and talk to judges and exhibitors.
Be aware that many catteries out there are in it just for the money, and as a result, they produce inferior cats. So be on the lookout for sickly kittens who will no doubt be prone to a lifetime of health problems. Before you get to the cattery, go ahead and make a list of questions to ask. Most breeders will be glad to see that you're responsible and have the cat's best interest at heart. Once you're there, pay close attention to the health of the kittens and the living conditions. Does the breeder seem concerned about the kitten's future home or more interested in making the sale? Be sure to find out how the cattery handles contracts. Most catteries include contracts as part of the transaction. And it's a good thing because it usually includes details on their return policies.
Another option is to find a Persian rescue organization. Rescue groups keep Persians that have been abandoned or lost. You can also check with your local humane society or animal shelter - sometimes Persians end up there. Not only would you be getting a new kitty, but you would also be saving a life.
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Regardless of where you get your cat, you may want to pick up a book/guide on owning a Persian. It can't hurt to bone up on Persian health and grooming.
Adapting to a New Home
You can get a wide range of responses from cats adapting to a new home. Some cats will settle in immediately, while others will take months to really get comfortable. Crying isn't too out of the ordinary and should subside in time. (He/she is still unsure of the environment and is looking to you for comfort and safety.)
Other pets are another factor to consider. The older the cat is, the harder time he/she has welcoming new animals into the household. Cats are territorial by nature. So don't expect things to change during the first few weeks. After a while, things should improve. (Occasionally cat relationships don't get any better, and the cats end up only tolerating each other.)