Invasion, maybe? That's the first episode when the Mercy West doctors are at the hospital.
Meredith wasn't in the episodes around the merger a lot because Ellen Pompeo had a baby, so Meredith donated part of her liver to Thatcher. She was still in the hospital recovering in Invasion and I Saw What I Saw, then recovering at home until she came back to work in New History.
Yes, Tainted Obligation.
If you have Netflix, the first 12 seasons are on there. Otherwise, there are no legal ways without buying them on Amazon or iTunes.
There are some clips on YouTube, too, but it's hard to find what you're looking for and there aren't full episodes.