I've bought mine at Jones New York. I've always been lucky to find one of the handful of outlet stores nearby, which seem to carry all the traditional suit styles. I like JNY as you can buy the jacket separately from the skirt/pants - and thus, if you need a petite jacket and regular pants (as I do - which is funny, as I'm 5'6") it doesn't cost extra. And the quality is very nice, with good fabrics and cuts.
I know people who like Neiman Marcus, as they do alterations (free?), but I haven't tried them yet.
When I was young, I just went to Brooks Brothers, but now, ugh. I find their stuff too stodgy. I may be older, but I don't want to look ancient.