I began MRing by checking the FT Mileage Runs deals. I was amazed how frequently YYZ & YUL shows up.
Now I sub to Soctt's Cheap Flights. Its worth the cost. They post 4-5 x/day and again, it was surprising to me to many originate in CA.
Beyond those two, I keep an eye on what Star Alliance partners are doing. In December, for example, I am doing YYZ-AMS on TAP for $550 CAD. Free stopover in LBO.
Looking ahead to the future, the AC takeover of AT is going to reduce dramatically the # of deals out of CA airports. In the past, I have done repositioning flightsto pick up MRs in IAD, EWR, and even ORD. Surprise, surprise some of them are on AC metal. That works for me...........I just skip the last segment.