Sources for purchasing parts

[Glen]

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!!

[amecks]

  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