Don't know where you can get the foam, but I have some questions you might want to ask yourself. I am sure you have thought about most of them, but you may have missed a few:

1. Was that TV show in a place that freezes? What happens to the foam exposed to the elements? Would it be better to use a landscaping adhesive made for that application that would surely last? 2. Does it carry any toxic compounds for the soil or worse, for the pond? 3. Do I really want that foam displacing soil that plants could grow in? (I suspect for most gardeners, It wouldn't matter, but it would be disastrous for an alpine garden.) 4. If used in a wall: does it matter that foam would impede water flow through the wall? 5. Can I remove the foam if it gets on the face of rocks?

It seems like such an easy fix, but I'm sure you can tell I am skeptical. Good Luck!

We use the grey foam all the time in ponds with fish. It is not toxic and it doesn't leech out into the water. Unfortunately I have only been able to find it my irrigation or pond supplier. It costs about $9 a can. Where as the white stuff from HD is about $3 I would sure like to find another source as well.

What is the foam called? I could use some in a project I am working on. RJ

Home Depot has black foam which does the same thing. I bought a can yesterday. They keep it on the shelf with the pond supplies.

Hi,
don't know whether it's just the Spanish quality of foam, but here I used (not for the garden) a filling foam from the Hardware store which comes out white (and costs MUCH less than specialist gray foams) but after a few days turns to a grungy brown color. Perhaps it's worth a try with your local brand to see whether the same effect happens with the USA variety?
The colour would be preferable to the white for the garden(though NOT when you, like me, WANTED it to be white!-though a few flicks with the paintbrush restored the desired whiteness for me) Good luck with it anyway...sounds a worthwhile idea!
BobSpain