You are mistaken. Plusle is in the same egg group as the two in question. Because they are both in the Fairy egg group it is possible to breed a female Plusle with a male Pokemon from the Fairy egg group. Which means to breed a Plusle you would first need a female Plusle.
See here for more information.
Your not out of luck though. In Black 1 and 2 after game completion you can come across swarms of certain Pokemon that are not native to the region. Plusle can be found on route 6 in swarms once every so often.