without good connections it is hard to become a independent bottler. But better still is it to have a distillery of your own! In this case you can exchange barrels and you do not have to pay money every time you need new whiskies for the blends you usually make.
That is the reason that more and more independent bottlers aquire distilleries and in many cases that is a good thing in itself, too.
Some examples are Cadenhead and Springbank (Sprinbank bought Cadenhead in this case) Gordon&MacPhail and Benromach Murray McDavid and Bruichladdich Ian MacLeod and Glengoyne and of course Signatory and Edradour.
Probably there are very old connections and contracts and as you rightly say some distilleries have licenced their bottlings to independent bottlers or we would never have tasted their whiskies at all.