I often sleep in underwires. They still last for 5-6 years "new", and then start to lose it on the 6th/7th year.
My other fav, is a new find. It's an underwire "tankini" from Norm Thompson. My mum gave it to me some time ago, but even when I was thin I had issues with my stomach, so I never swam in it. It's not the right size for me (38ddd is mine, it's a 40d) but it is my FAVORITE new thing to sleep in. Girls stay where they belong, and I can get out of bed not looking like I weigh 60lbs more than I do / hurting because of the dangling participles.
When I'm on a weightloss kick, I wear a sports bra (underwire) to bed. Partly because the only way to get me to exercise in the morning is to already be in my gym clothes, and partly because it helps the skin recover as I'm shedding inches. ((My fav is the 'Cuz She Says So' from Title9. They also have non-underwire models))
And warning... as you said 'big enough to sit on'... by the time my son was born I was a J+, slowly came back down to a DDD by the time I was finished nursing. So don't drop a bunch of money.