I think the ones connected to the Valyrians, like North Valyrian ruins or Summerhall. And as for the event to buy an egg, it seems that the ambition isn't actually required, but it has a MTTH of 75000 months and only three eggs in total can be received in this way throughout a playthrough, as well as requiring 800 gold. If none have been sold and you have the ambition, MTTH drops to 30000: 2500 years. Being fire-obsessed cuts the time in half while being just, honest, and honorable cut 10% off each. Being in Asshai cuts the MTTH to a quarter if you're willing to wait that long. If you match all those, the MTTH is 2733.75 months, or about 227.8125 years. If you leave Asshai, lose a trait, or an egg is sold, I assume this resets the timer and increases the MTTH, so I wouldn't count on getting an egg through pure chance.
that's not hwo MTTH works - it's not a counter, there's a small chance of an event to happen base doff the MTTH*modifiers.