This is the database I like to use that has all the unicode characters. Some are supported by Sporcle's font and some aren't. Some will be visible on everyone's computer (e.g. the chess and card suit symbols generally work) and others won't. Here are the categories I've found the most useful:
Miscellaneous symbols (most of which will work for everyone… but don't quote me on that) and
Miscellaneous Symbols and Pictographs (which are hit and miss - and show up as colour pictures when they do work). You can click on any of them to see what they look like on your browser and to get the unicode information. Copy/paste seems to work okay with most, but be sure to test your quiz.
In this quiz, I used three symbols that were only one number apart in unicode, and several people found that two were visible and one was not (showed up as a box)… You can read the comments to see what people saw.
Unicode is probably your best bet, but like I said, some characters (especially further down the list) are not supported.
Good luck! :-)