Welcome to the Forum!
Why are you looking to build a collection
before you start? As a Disney employee you will have access to the most sought after pins
after you start. Not to mention your employee discount on regular pins. So if I was you, I would just dive in once in the job.
At that time, you will have access to pins only Cast Members get access to - specifically, Disney Employee Center (DEC) and Walt Disney Imagineering (WDI) pins. They generally each do a release a month. They are available online with heavy competition so you may or may not manage to get pins from any given release. I suggest that if you do manage to get any, get any/all that you can. Because they are limited in distribution and quantity (generally Limited Editions (LE) around 300 or so) they are in high demand and you will be able to easily trade any you don't want for great pins that you do want.
There is also an employee store where pins are sometimes clearanced and could be a great place to great traders on the cheap.
Enjoy your new role and collecting!