Indian ringnecks are a fairly common species of pet bird in the U.S. and so are fairly easy to find. You can either adopt/rescue, or purchase from a breeder. You can expect to pay anything between $300-$700. If you choose to rescue or adopt, make sure you use a reputable organisation such as Rescue Parrots.
If you choose a breeder, check that they have experience in raising and breeding parakeets and are knowledgeable about the breed. Either way, you want to make sure the bird you choose is healthy. Always check for bright eyes, well maintained plumage and a full crop.