Male cockatiels over one year of age lose the horizontal bars over the underneath of their tail, whilst female keep them. The vet will be able to advice you further on this. To be absolutely sure of the sex of your bird, there is also DNA testing via a blood sample or surgical sexing by a vet. Your vet will be able to advise you about this. In terms of finding a mate for your bird, you could contact local breeders to see if they have any birds available, or find a hobbyist club to join.