Harbor Freight actually sells the replacement foam online. Someone provided the specific link to it earlier, but I can't find it.
I'm sure if you go to the HF website and do a search, it will come up.
As many have said (and I agree), the foam it comes with is really cheap. Some have tried using the plastic dip method to help reinforce it and to keep it from falling apart, but do yourself a favor and do what I did.
I went to this site and bought some layered foam. It is a different closed-cell type foam. Not sure what it is called but it is a hundred times better and very easy to make custom cutouts.
Easily create neat ''French fitted'' liners for your drawers—layered foam lets you peel out thin layers until you reach your desired depth.
www.rockler.com
You can also find it on Amazon.