I would like to get a partner for my cockatiel, but how?

So, unfortunately, one of my birds died, and my cockatiel is now all alone. My parents are also in favor of getting him a mate, but I don't know how, and I can't find a suitable one. My cockatiel is 5 years old and tame. I'm looking for a mate of roughly the same age, preferably a female. But the problem with his age is that I can hardly find any older cockatiels. And I don't know what the older cockatiels have been through or how they were raised, or whether they're even tame. Older birds are harder to tame. Do you have any ideas?