I wasn't thinking that propane comes from oil, but it can. If I would have remembered my chemistry I would have realized it: C3H8 is propane; it's a hydrocarbon!
I imagine 'they' can obtain propane from nearly anything with an abundance of C's and H's, sobit also can come from other fossil fuels/natural gases. I think most of it comes from these sources, but apparently not all of it.