For a lonely bird, the same species is always your best bet for a friend. If your ringneck needs a companion, I would just go for another ringneck, but very important: two birds, even of the same species, do not always get along and if this happens then you need two separate cages of proper size, and twice the perches and toys. Be prepared for them to not like each other.
If you mean you are trying to find for a ringneck for sale, then feel free to post on the Birds Wanted forum and see if anyone has a bird ready to come home with you!