Like all Pokemon, Ditto spawn randomly. Their spawns seem relatively low (compared to the Pidgey and Rattata they like to pretend to be). Personally, I've only caught 4 since they were made available. It might be worth noting that they were all caught in the same place and that I haven't caught any elsewhere, though I haven't really gone on Pokemon-catching walks since long before Ditto were out.
That said, it's not yet confirmed whether or not Ditto can have nests. This post says yes, but there is not enough evidence to determine this conclusively.
Can confirm Ditto spawn points. Caught two 2 hours apart, first being a Pidgey, second a Rattata, from the exact same location.
Basically, since Ditto are so new and aren't readily visible/observable, there is much that is not yet known about them. That said, your best bet to finding a Ditto is to catch everything you see.
Another helpful tip is that if a Pokemon is exceptionally and uncharacteristically difficult to catch, it's somewhat likely to be a Ditto. For instance, the past two Ditto I've caught, I figured were Ditto prior to being revealed. The first was a mid-range CP Rattata that escaped two razzed Excellent Great balls and the other was a mid-range CP Pidgey that escaped three Excellent Pokeballs.