That depends on where you are. The Smashboards tournament section is a good starting point for a general look, and I mention that because a lot of tournaments found there are posted by board members that find the information from outside the site. You can use that section to figure out where they get that data and then look there on a more permanent basis.
I did that for tournaments where I live. I found out that the reason I couldn't find a website or forum with information where I lived was because it was actually completely hosted on Facebook, and board members were posting tournament info from there to here in order to spread the word. I actually re-opened my Facebook account to keep an eye on when new tournaments are formed to get a jump on registration.
TL,DR: Keep your eyes peeled and ask around for info on your specific location helps. (State, Country, etc)