How do I determine parts for my bike and where to source them (new)? Do bike shops have specs guides or manuals to know which (eg: integrated headset bearings) parts to use on which bikes? And where do they source said parts (other than from the bike mfr)?

And most importantly: where can the average Joe like myself get this info and buy parts?

The bike I'm currently working on is a 2011 Scott SUB 30, but I can't seem to find any information about specific componentry online. If I were to take it into my LBS I'm sure they'd have no trouble figuring out where to buy, eg, the correct drailleur hanger or headset bearings.

How can I do the same? Important note: I live in Canada, so would prefer not to pay large import taxes from international sources if possible
You should look your bike up online, you should be able to find a stats page that will give you some general specs.  The shops I have been in do not have a "manual" per se of what frames conform to which standards.  It's generally done through experience and visual confirmation/measurement of the bike in front of them.

Derailleur hangers are generally purchased from the manufacturer, but there are some aftermarket ones that will fit specific models of bike.  

Bike shops buy parts from a wholesaler (like QBP or J&amp;B).  Most wholesalers won't deal with an individual unless you have a storefront (business) and $X amount of insurance.  You can find businesses online (Colorado Cyclist, Bike Nashbar, Performance Bike) that sell parts online, but realistically you are probably better off buying from your LBS and developing a relationship with them.  Oftentimes, the prices at your LBS will be similar to online without the shipping.

Derailleur hangers are one of the few things that are manufacturer specific.  Most other parts conform to some standard and your average mechanic or shop rat will be able to look at your bike and determine what type you need.