I had a cinnamon- pearl Female "Sunny" which died about a year ago from being egg bound. She was in a huge cage with 1 other female (lucy) and 2 males (Skye and Casper). I then rescued another male (Doodle). The 3 boys started to get a little to friendly with Lucy, so I separated her. Lucy was very unhappy being alone. I too let her have supervised out time with the boys, she loved that but was still so sad while in her cage. It just so happened that my best friend has 2 females and 1 male cockatiel and she was having the same problem. We decided to "trade" birds. She took Lucy and I took her male, Max. It worked out great, Lucy is so happy and I know she is well cared for and I still get to see her. Max is awesome, he was a rescue of hers that I had even named (when she got him) after a character in a book.

With all that said, I would say that another cockatiel would be a great idea. They are such social birds I'm sure he misses having a companion.

I have 3 rescued cockatiels none of which are hand tame. My only hand tame cockatiel is Casper, who I had bought. I find that in most rescue cases the owner didn't have the time for the bird and therefore they aren't hand tame.
I would say getting one from a pet store or a breeder would be best. But be careful buying one from a pet store too. I have seen pet stores selling "hand raised" birds which is different from "hand tame". Often "hand raised" means the babies were in a box and getting a syringe shoved in their face several times a day, never actually having hands-on contact. That's not everywhere, but I have seen it. Of corse be sure to check for cleanliness.

Good luck!!