I use the makita ones, got them on Amazon. I actually use them to connect the makita rail to a festool rail [eek]
I know people say that you need 2 sets, but it's actually recommended by Makita to use only 1 connector to connect 2 rails. And I've never needed to buy more than 1 TBH. Maybe if you're constantly moving it around all day, you may want the extra insurance of 2 connectors..