1. google "decorative plumbing pipes" I got all kinds of hits, or go to a plumbing store that sells free standing or wall mount sinks and ask, they will carry them. 2. yes and no. depends what you are looking for, they come both ways.

I have examined your photo in detail and find a number of issues that would be code prohibited today, however given that we have not installed oakum & lead joint cast iron in the last 40years it must be assumed that it is all original to the structure and that it did pass code at the time of installation therefore you are not obligated to upgrade it unless you make modifications to the fixtures or piping in the downstairs bathroom. On the other hand, since the downstairs bathroom is venting off that stack you would not be permitted to discharge a watercloset into that stack on the second floor unless you first establish an auxiliary vent for the first floor bathroom. The problem here is that the line to the watercloset is a 4" line therefore it would require a 2" auxiliary vent.

greg_2015, I'm really glad you got back to me on this. I'm not a plumber (obviously) but figured there had to be a different place where the plumbing is vented. But since I'm in a condo building and on the top floor, I did not want to take a chance and simply go ahead and cap the pipe. I even went as far as to consider that maybe the sink bowl holes are the vents, especially when I saw the black holes in the black pipe (see photo in my first post). After I got your response I went onto my deck, which gives me a clear view of the roof, and indeed saw what I consider to be the vent stack. By examining its location and the location of my kitchen and bathroom sinks, I could clearly see that the vent stack it right above the sinks. The photo below is of the vent stack. While standing on my deck I can touch the edge of the pitched roof with my hand. In fact, just below the photo is my deck door which leads into my kitchen. So the distance between the stack and me, from where I took the photo, is quite close. And here's a photo of the bathroom sink pipe after I capped it: By the way, something strange has happened. My kitchen sink clog seems to have gone away! This may have happened before I even capped the bathroom sink pipe. After I capped the bathroom sink I ran water down my kitchen sink which is on the other side of the wall from my bathroom sink. I was expecting to see the water back up into the kitchen sink. But no, the water kept draining away. Is it normal for some sink clogs to go away on their own after a number of days or weeks?

Cavity or insulation I really have no idea. Outlets and switches are in the wall. Unless plaster walls are really that dang hard and solid feeling. But thanks for everyone's replies, think I have some more investigating to do.