A few questions for those with experience. I have been dismantling, cleaning and repairing power tools for fun for just a few months. The drill I am working on now may need new brushes. Here are my questions:
1. At what point do brushes need to be replaced?
2. Where are some sources to purchase replacement brushes?
3. How can I ensure that I am ordering the correct size / part?

Thanks in advance!!

  I repaired many electric tools when I worked for the rental company.  I was usually able to order parts directly from the manufacturer's website.  Ordering with the tool's model number and serial number will assure that you get the correct part.  Read everything in their descriptions carefully.  Some of these tools I fixed were older tools but none of them were "vintage" tools. 
  Finding vintage parts may take some creativity on your part.  Research the tool to see if it was made under a different name brand.  Use the the internet to find companies that specialize in parts sales for tools.  Ebay can be a source for older parts.
Al

Eurton Electric Company in Whittier, CA   https:/ eurtonelectric.com

I usually start out with Ereplacement parts!

Look around for motor repair shops, tool repair shops, and the like; but first, stop by your local hardware store (or, if you have more than one, the oldest one).  Our local store carries a selection of brushes and even portable power tool switches; eventually, I'm going to take the switch from my Blackened Drecker 1/2" drill and see if they have a match.

Yes. Same here; eReplacementParts.com. I would not know what to do without them.

But this site is Classical Power Tools and for those, I'm almost entirely met with the words, "Discontinued" or "Obsolete" or "This part is no longer available"

Only, if the part is shared by other current or at least more recent models, am I met with success.

As far as Carbon Brushes and such, which were left behind in the past, it seems you have to  measure and visually categorize em, so you can match em up with an inventory; i.e eurton.

Stuff like Part Numbers are of little or no use.