Without the GTS, it's pretty difficult to complete the National Pokedex in any game that came out before X and Y, but it's still possible. Despite the GTS not working anymore, you can still trade locally with Black, White, White 2, and other copies of Black 2. You can also transfer from Diamond, Pearl, Platinum, HeartGold, and SoulSilver. The only downside is that both of those methods require another system and at least one other game, so if you don't own other games and/or another system, and you don't know anyone with a game/system, you can't transfer or trade. If that's the case, then there's not much you can do. The only other way (That I know of.) that you can get more Pokemon that aren't accessible in B2/W2 without use of another system is the Dream Radar, but, unless you coincidentally are missing only the Pokemon that can be found in the Dream Radar, won't get you very far.
Hope this helps!