For a big race like Flanders, and for most of the Belgian spring classics, the streams will be plentiful and start broadcasting earlier than most other races via the main broadcaster (Sporza - although you don't link directly to their site). As long as you dont mind listening in Dutch, which I often find prefereable to the inane British announcers on Eurosport.
I like using steephill.tv to access the links, because they mention which streams are georestricted so I don't waste time waiting for sites to open and see some version of the message "access denied."
As other posters mention, people will always post working links in the race thread, but some will not work in the US.