Is ditto only available during the Thanksgiving event?

Ditto is in game now permanently. Just caught one now. You can often tell when it is ditto as the pokemon is hard to catch and often breaks out. If this is happening perhaps try a great ball on the pokemon.

Caught two dittos today so no it's not only during thanksgiving event

Still available - I'm averaging 1-2 a day.

Seem to favour busier locations - local bus station has provided three.

hey sorry to hear that, I caught 3 in about 7 hours of catching everything I saw (over 3 days).
The first Ditto had no giveaway signs. The other 2 broke out more than I was expecting.

I have caugh more than 20.

I haven't found one in days and there is a spawn at my house where I have caught 2 in the past. But haven't seen one in a long time.

The only thing I've seen that I think is indicative of a ditto is, on a couple of occasions, I've had a Rattata break out of a ball and immediately attack me, but halfway through the attack animation he runs.

I've never seen that behavior before so I'm guessing it's likely that it was introduced as part of Dittos being added. Of course it's nothing but speculation, and doesn't help you catch one, sorry.