If you're using Realtor.com, all listings should appear there, and pretty promptly, too. Trulia, in theory, should have them all though my first choice would be Realtor.com or your local MLS. And Realtor.com and your local MLS probably will have them quicker than Trulia.
Really, you shouldn't be missing out on many properties that way. What you'd be missing out on are pure FSBO properties--properties beings sold that are never listed on the MLS. There will be some, but likely your agent won't have easy access to them, either. There are some sites that are set up specifically for FSBOs, though, so you should check those, too. And some FSBOs sell on sites like Craigslist.
So: Best online resources would be Realtor.com or your local MLS through a local Realtor. Then make sure you're looking at the FSBO sites and at Craigslist. That's about as good as you'd be able to do.
Hope that helps.