You don't even need to buy them in bulk. Most stores will have a bunch of free commons from players who are playing draft or sealed in a big box. Just grab whatever you want from there, most sets are going to have at least a few goblins, undead, and beasts.
But yeah buy a 1" hole puncher, some 1" washers from a hardware store or a 1" wooden dowel cut into smaller pieces. Or here 100 1" wooden circles. Get that, a 1 inch hole punch, and some glue.
Edit 2: The guide is available here
I made these token mini's as an alternative to buying miniatures. They're significantly cheaper and can be made in 0.5", 1", 2", 3" sizes and cost very little (1" and 2" shown).
If enough people are interested, I'd be happy to create a DIY post or video.
Edit: I created a MS Word template (link) for 1", 2", 3", and 4" tokens for anyone that doesn't want to wait until I post my guide.
I use wooden circles as the base - they're available on Amazon in 1", 2" and 3" sizes; /u/ScoutManDan suggested MDF discs.
The tops are epoxy stickers - they're also available on Amazon in various sizes.
When it comes to assembly, this is the order I use:
I did this for Tomb of Annihilation, check it out. Here are close up pictures. I glued them to wooden nickels, which you can find in 1 inch, 2 inch and 3 inch sizes on amazon. You can make placeholder tokens for extra enemies in groups.
I used these and they work well but the other posted Amazon link looks a few dollars cheaper.