Summarize:

Faded, torn, or patched jeans will work if the rest of the outfit is sufficiently hippie, but the holy grail of hippie bottoms is none other than the denim bell bottoms. Men and women both wore these; they're a staple of hippie culture.  Embroider a peace-sign patch on your bell-bottoms. Denim, corduroy, or patterned pants will work as long as it has that characteristically hippie flair at the bottom. In fact, these might be a good alternative to your best pair of denim bell bottoms. Having trouble finding bell bottoms in this age of the skinny jeans? Here's a tutorial on how to  turn a pair of jeans into bell bottoms. Or better yet, rehab your old ones by fraying, ripping, or tearing them (although it's not strictly necessary). If you're male, turn your old jeans into cutoffs. If you're female, try turning your old jeans into short shorts. Really, the grungier, the better, especially if you're a dude. You're not really concerned with your clothes being pristine and pressed. There's much more that you can do with bottoms as a female. Take advantage of it! If the weather allows, you have many options. Consider:  Loose, flowy skirts (think gypsy style) Dresses such as tunics or sundresses Even mini- or micro-skirt (especially if paired with above-the-knee boots). Many male hippies did wear robes or even skirts. There are skirts made especially for men. Don't be afraid to wear what feels right regardless of gender.
Go for denim bell bottoms. Get some denim shorts, especially if they're ripped or torn. Utilize your options if you're a lady.