As no Kanto Pokemon were retconned to the Dark type, and FR/LG uses the Kanto Pokedex until the postgame, there's no way you can get a Dark type legitimately without trading. The only Ghost type legitimately accessible without trading is Haunter, which isn't exactly the best (plus it's a Poison type). In short, you're out of luck if you want a Dark type Pokemon.
...But, what about Dark type moves? Your Raichu, Nidoking and Mr. Mime can learn Thief (TM46 in Mt. Moon), and Ninetales can learn Faint Attack via breeding. Vaporeon learns Bite at Level 30, which is probably the best (lol) Dark move you can get with your current team, as Thief isn't very good. With solid Special Attack and Special Defense, Vaporeon can use it fairly effectively.
It should be noted that Persian is also rather easy to get and can learn Bite, Thief, and Faint Attack. However, Persian's Special Attack is subpar, so Vaporeon may be the better play. If you choose not to use the Mr. Mime though, Persian is a somewhat viable alternative with slightly below-average stats. On the bright side, it easily gets several Dark type moves via level up (and Slash).
The only Pokemon that learn Crunch in Gen III are Nidoran (F), Nidorina, Growlithe, and Rhyhorn. Growlithe and Rhyhorn must breed for it, while Nidoran (F) and Nidorina must wait until fairly high levels to obtain it. This typically isn't worth it, as Nidoqueen is available much sooner and is in most cases much better.
