Should we or should we not allow (and answer questions) that ask how to acquire a certain chemical?
Ignoring the legal/safety issue for a moment, it seems to me that this kind of question is a little like questions asking for textbook suggestions, i.e. they can generally be quite interesting and spark a debate, but ultimately don't yield answers that will be useful in the long term/to anyone else other than the person asking the question.
Consider some imaginary request of
where can I buy sodium?.
It's easy enough to google 'buy sodium online', you can imagine a similar situation of someone asking
what is the pKa of ethanol? where the question would likely be closed/the person told to go google.
Most answers that actually address the issue would likely be country specific, which seems less than useful to the community as a whole. We also seem to discourage answers which point people to websites which may/may not be accessible in the future.
Getting back to the legal/safety issue:
The fact someone is asking this question suggests that they aren't in a position to obtain the chemicals 'legitimately', that is, they probably aren't at a high-school/college/university where they would be able to carry out practical work. Whilst I've been drawn into these discussions a few times, I tend to eventually get very concerned that the person asking them has very little idea of what they're doing beyond watching a youtube video where someone shows them how to do something.
The man in the video drops sodium into the water, I'm very safety conscious, I'll stand next to the hose
Despite the question stating 'curiosity' etc., we actually have no idea who the person is/what there intent is. Obviously theres some common sense, but certainly in the case of anything remotely pyrophoric/flammable etc. I'd have concerns. If nothing else, we don't want Chem.SE to become advertised elsewhere as the 'go-to' place for finding out where to source things.
Chemicals are highly regulated in the UK outside of certain areas (academia/industry), and I know similar legislation exists throughout europe. Since we ultimately don't know where the people asking are from, it seems un-wise to give advice that could lead to someone un-knowingly breaking the law (in the UK even seeming harmless chemicals such as permanganate are highly regulated under drug laws).
In short, I think like textbook questions theres a good way and a bad way to go about it. I'd suggest that when these posts do arise, someone comments (as per homework questions) to request more details about what the item is for.
Edit: To include Jan's
Where can I buy Iron?.
In the case of something that we wouldn't consider harmful, I think points 3-5 can somewhat be ignored (or you may choose to ignore all 5....), although I would still suggest that we have no idea what the iron would be for and as such should exercise caution.
Even if we took an example which was 100% harmless, say
where can I buy distilled water?, I'd still argue that the question adds very little to the community in the long run. A quick search of google would reveal sources and hence theres really no need for an answer.