So I've been trying to complete the National Dex in White 2 for a while now. I caught every single Pokémon catchable in White 2, including swarms, hidden grottos, etc. Everything else I have to (try to) find on the GTS.
Here is every Pokémon I need:

Gen 1 evolutionary lines:
-Machop/Machoke/Machamp
-Bellsprout/Weepinbell/Victreebel
-Tentacool/Tentacruel
-Rhyhorn/Rhydon/Rhyperior
-Kangaskhan
-Jynx

Gen 2 evolutionary lines:
-Murkrow
-Slowking
-Unown
-Teddiursa/Ursaring
-Slugma/Magcargo
-Houndour/Houndoom
-Phanpy/Donphan
-Stantler
-Smoochum

Gen 3 evolutionary lines:
-Treecko/Grovyle/Sceptile
-Mudkip/Marshtomp/Swampert
-Zigzagoon/Linoone
-Cascoon/Dustox
-Nincada/Ninjask/Shedinja
-Mawile
-Electrike
-Torkoal
-Spinda

Gen 4 evolutionary lines:
-Piplup/Prinplup/Empoleon
-Cherubi/Cherrim
-Honchkrow
-Stunky/Skuntank
-Snover/Abomasnow
-Froslass

So if you happen to have any of these Pokémon in any gen 5 game, please let me know. My IGN is Dawn.

I've gone down my list in order to make sure I don't miss anything. The only time I go out of order is if I see an offer of something I need, then I will jump on it, in case someone else swoops in. (that happened to me yesterday- I was about to get a Slowking, but I was having internet troubles and by the time I fixed it, no more than 10 minutes later, someone already got the Slowking! I'm still mad!)

*all Pokémon whose names are italicized are not registered in my pokedex. The GTS in gen 5 doesn't allow you to type the name of a Pokémon not registered in your dex, so you would have to find an evolution and breed it.

Let's use the Snover and Abomasnow as an example- if I wanted Snover, I can't search for any offers of it because it isn't registered in my dex. I would have to search for an Abomasnow and then breed it.

In the case of Unown, Spinda, and the Treecko, Mudkip, and Piplup evo lines, I can't search for any of them, so it's very unlikely that I will get them in my dex which sucks because I can't get my favorite Pokémon in my White 2...

The only way I think I can get them is through something called GTS negotiations, which is a lot like wonder trade where you trade with a random person, but you can each pick up to 3 Pokémon to trade. For GTS negotiations, it doesn't matter if it's registered. The only problem is you have to hope that the person happens to have a Piplup that they're willing to trade. And almost every single person I've connected with just wants to trade shinies or legendaries.