I move alot of intra US and use Internet Truckstop. For the smaller LTL I use a Old Dominion, YRC, Fedex Freight, and R&L. Usually the hardest to cover are the partials, so I usually price these at near truckload rates and get a hotshot. All things considered I much prefer Canada US and would stick to that if I had to do it all over again. Most of my truckload freight is flatbed, so I can't tell you much about van or reefer. In my experience, so long as you're factorable you won't have a problem getting trucks.
One more thing, make sure your customer knows you're in Canada. Most tend to take it for granted that you're in the US.. nobody even questions that you might be one of the dreaded "farners".. A few years ago I had a carrier give me a lead on some US DOD freight.... moving armoured personnel carriers, tanks etc from WI to east coast ports. All went well until someone there woke up and noticed they were using a Canadian broker (me) That ended that deal pretty quickly, and I'm pretty sure someone at DOD got canned over it. So lesson learned.. make sure your US customer is ok working with a Canadian broker.. most don't care one way or the other.. but some will not.