I've never worn them so they are visible under boots- but they are great (not sticking out of boots) when it gets cold out for keeping your legs warm.
I've been seeing some people wearing them sticking out of their boots- and it can be cute, but it can look really messy and/or childish too- I think it depends how you do it.
and ya- target, H&m, urbn, free people all have tons (I like free peoples the best atm, but I think they are a little overpriced for what they are)