But for some reason I had a hard time doing so. The thesaurus didn't help, and I could only find one Antonym dictionary. Turns out that the opposite of "fear of closed spaces" isn't "fear of open spaces" but instead "lack of fear of closed spaces", which is, I guess rather precise, but not exactly what I was looking for.
The word I was looking for was, of course, agoraphobia. And my web based attempts at finding it were unsuccessful. It shouldn't have to work that way. Among all those portals you'd think that someone would offer you a link to a site that actually allowed you to quickly and readily find a word you've forgotten.