After finding Mesprit, Azelf and Uxie in Pokémon Black 2, things you will be able to do would be going to N's Castle in order to battle him and defeat his Zekrom. Another option is taking the Dark Stone after beating N to the Dragonspiral Tower in order to reawaken Zekrom in order to attempt to catch it. Other options would include attempting to catch Cresselia, Heatran, Regirock and Regigigas.
No, you will not be able to find Mesprit in Pokémon Black, you would have to trade with Black 2 or White 2 or you would have to transfer Mesprit from a previous game.
You can get them at the lake's. they are lake verity lake valor and something else. mesprit is a roaming pokemon, which means that it roam's around sinnoh, and you can track it's location using the marking map pokeketch app.
Mesprit & Cresselia (legendary)
your home lake
In map 22 (Ground)
mesprit is at the descon wold and then it will be at verity lakefront and then it will start going around sihhoh in Pokemon platinum
You'll find The Lake Trio: Mesprit, Uxie and Azelf.
uxie azelf mesprit and palkia
By pooping your aself
You can find Palkia, Uxie, Mesprit, Azelf, Giratina, Cresselia, and Regigigas
go to all three lakes and you shall find all three
First you have to go to the lake closest to your home town. Use surf and go to the little 'island' in the middle. enter and you will see mesprit. Go talk to mesprit and it will automaticaly flee (don't worry, you and mesprit just start playing a game of... 'tag'). All you have to do is look for mesprit with the Pokemon tracker and battle it. It's best to have a Pokemon that knows mean eye so it cant escape.