I just train all my Pokemon to level 20 by now. XD
If you're too lazy to take the time to train your Pokemon, you could just catch a Growlithe outside of Violet, or in Union Cave, you can catch a Geodude or an Onix. You could also trade a Bellsprout for an Onix in Violet City. If you're close to nighttime, catch a Hoothoot, you could also counter back with your own Bug-type, even though I don't recommend that.