Here are some Electric types:
Eelektross. You can find wild Tynamos in Chargestone Cave which is about mid game. I think Eelektoss is one of the best electric type Pokémon you can get in-game. It has good attack stats (115 and 105), and learns lots of good moves like Flamethrower, Crunch, Brick Break and Flash Cannon.
Galvantula. Joltik can also be found in Chargestone Cave. This one is easier to get because it doesn't evolve via stone. Galvantula is fast (108 speed) and has pretty decent special attack (base 97) too. It won't be taking any hits though, its defences are pretty terrible.
Zebstrika. Blitzle can be found on Route 3 which is really early on in the game. Zebstrika has good Attack and Speed, but like Galvantula, it has terrible defences. Zebstrika doesn't get many good physical moves outside of Wild Charge, Flame Charge and Stomp which kinda sucks.
Stunfisk. Now before everyone yells about how Stunfisk is literally the worst, let me explain why I'm listing it. It has pretty good HP and Special Defense, and decent Special Attack too. You can find Stunfisk pretty late game on Route 8, Icirrus City or the Moor of Icirrus. I've used a Stunfisk in-game before and it wasn't really that bad. It gets some good moves, and even learns Surf which is a good HM move to have.
Stunfisk is also the only remaining Electric type you can get without trading that doesn't suck. The only other option is Emolga, which is basically a worse version of Galvantula. If you can catch an electric type like Mareep, Magnemite or Jolteon in another game then I'd recommend trading one of those over to your copy of Black to use.
Hope I helped!