The logic behind these facts is that "If I couldn't buy/find a N900 in the most convenient way, it is discontinued". You'd like to check the fact yourself in this case.

I can still order N900 in Hong Kong. Most of the shops here don't take stock of N900. Of course, they would rather stock newer phones with higher profit margin.

So when you hear a salesman tells you something is out of stock, it probably means "it is old and nobody here has any knowledge with it and I don't want to waste time on stocking something with low profit margin so choose something else or get lost I've more gullible customers to serve"