My question, 'Where can I find a source of Schlumbergera russeliana', was closed because it was thought to be unconstructive. What was unconstructive about it and why is it off topic?
The StackExchange network generally frowns on "shopping" questions. There is a famous blog post about this from the Stack Exchange team and it's called Q&A is hard, let's go shopping.
Now this can be a grey area. I had a short discussion with fellow moderators behind the scenes about whether Where-to-find-Schlumbergera was a shopping question, and what kinds of shopping questions we are allowing.
Basically the problem with your question specifically is that if the accepted answer today is Sid's Schlumbergera Store in Sacramento, then
1) Sid might go out of business next week and so the answer goes out-of-date quickly
2) it's not much use to anyone who doesn't live near Sacramento.
Tea Drinker's answer is the official policy, but I think it is possible in some circumstances to give a long-lived answer to some "shopping" questions. I have no idea how to answer the Schlumbergera question, but I think my answer to the seed-source question offers a long-term strategy instead of a possibly-short-lived "source".
I think that if the community here makes an effort to answer shopping questions with strategies rather than quick "answers" the quality of the answers is higher, and they age much better. (I guess this means we have to answer the underlying "how do I find a source?" instead of "what are the current sources?" that such questions are usually literally asking for.)
In other words, it's not that the questions are bad, it's that they have historically drawn bad answers. Let's not punish the asker -- let's try to rephrase the question so that it elicits good answers.