Pray tell...where would a person obtain borax and iron filling?

It's pretty cool, if you can find all of the ingredients. The bearded science guy offers these tips:

Sticky Slime: Dip your hands in the borax water solution and work with the slime. Sticky slime means you don't have enough borax.

Tough Breakable Slime: Add more water and work it into the slime. If this doesn't fix it after ten minutes consider adding more glue. Tough slime means that there is too much borax compared to water and glue.