Bigger stores like lever arms, frontier, wolverine will occasionally offer milsurp.
For the real deal here's everyone I can vouch for
P&S Militaria
Trade Ex
thecanadiansoldier dot com
He Who Shall Not Be Named
oldguns dot ca
There are also a number of smaller professional sellers selling through classifieds, and those that operate on the EE.
I've been aware of collectorssource dot com for a long time but I never bought.
And I recently started greatnorthgunco dot ca! It doesn't even show up on google yet.
But the real key to getting milsurps is knowing the right people. You get contacts by hanging out with people with similar interests and through trading. You'd be surprised to know how many collectors are in your town! I've made friends by buying and selling locally.