Eurton Electric is a great source for brushes, bearings and motor parts. Do you have the original brushes? I believe they should be 1/4" x 1/2" for that saw, but its been a little while since I last had an older 77 apart.
You can also get new brushes on tool repair sites like
Toolpartsdirect and you can often find generics on
Amazon. I also used to be able to find replacement worm drive brush kits at Home Depot, but with all the shortages going on right now, they might be a bit more difficult to find.
If you need brushes right away, I would just take your old brushes to a local tool repair shop and they will likely have a new set on hand.
Just my two cents.