I tried asking a question at superuser for some advice on finding a computer expansion card meeting some specific requirements, but was marked offtopic. Which site is the appropriate place to ask this?
The question was here.
The definitive list of sites is at http://stackexchange.com/sites, but that doesn't mean that there's necessarily a place for your question. Always be sure to check the FAQ for a site to make sure that your question is a good fit before posting there.
Looking at your question, another strong possibility is that it was closed for being a shopping-list question. (The typical close reason would be "Not a real question", but that's not ironclad.) You might try asking about whether such a hardware question is really off-topic on the Super User Meta, and, if not, rephrasing your question to be more objective. Something akin to "Is there any card that exists with these specific features..." instead of anything like "What's the best card that has...".
While Stack Exchange covers many topics, and covers them well, there is not yet coverage of everything. You could try Yahoo Answers, Usenet, etc., but some questions are simply hard to find the answers to.
If you're in a bind about a piece of computer hardware that's borked or needs amping with its own caffeine hit, then Super User is the place to jones.
But if that problem is you need to find a product in a sea of catalogues, your luck is out and your question is off topic. Not just on Super User, but across the Stack Exchange network.
These are both quickly dated and specific to only that user asking.
If you can reword it to elicit something of a guide in building components and/or general enough to help a greater audience, then it's usually a better shot at staying on.