The following are the current registries that recognize Ragdoll Cats:
The Cat Fancier's Association (CFA)
The International Cat Association (TICA)
If you see a registry other than these, it is not a real registry.
Limitations of a Registry:
Registries are meant to track cats. The information they have is about as good as the information supplied by the breeder. Which means, if the breeder falsifies a record, then the information is no good. Having a registered cat does not mean you have a Ragdoll, it means that a registry is tracking the pedigree of your cat; if you want to know whether your cat is a Ragdoll, you need to get its pedigree as far back as you can, and then hope that there are no backyard breeders who may have falsified pedigrees in earlier generations. Or breeders who just didn't understand what they were doing and incorrectly registered cats as Ragdolls when they clearly weren't.
In order to be registered as a Ragdoll, a cat only needs one Ragdoll parent. The problem here is that if a cat has a parent that is not a Ragdoll, then the cat is not a Ragdoll. But hopefully the pedigree can tell you this information, so that you are informed.
Breeders that don't register kitten litters...are probably not producing Ragdolls. As of the time of this writing it costs $20 to register an entire litter of kittens, and either a single form online or in the mail. It's not expensive or complicated, so those who do not do it are likely backyard breeders and know they are not producing true Ragdolls.