Where you get either Elekid or Magby is up to whether you have either the original Black and White or the sequels.
- If you're playing the originals, they are found in White Forest after you've beat the game once (as you're unable to go to White Forest until you do so).
- If you're playing the sequels, both can be found in the Virbank Complex, with Magby in Black 2, and Elekid in White 2. Other than there, they can still be found in White Forest as before.
However, if you're REALLY impatient, just use Poke Transfer, although you still have to beat the game once before you have access to it, and in Black 2/White 2, transferring to get them isn't as good as it was in the first games, as you already have access to them, unless you have a perfectly capable competitive Electivire/Magmortar in Gen IV that you want to try out in Gen V.
Hope this helps!
Source: The Inner Workings of My Mind