Some kind of foam is helpful and really necessary for needle felting. The photo here shows a yoga block on the upper left, a thick piece of chair cushion on the right, and a thinner piece of foam that came with a computer I bought as packing.
The foam supports your work as you felt. The other purpose the foam serves is to protect you from stabbing yourself with the felting needles. Ouch. They are sharp. They are barbed and they really can hurt going in to your hands, knees, legs or whatever they are facing when they inflict their wounds.
I have many times been happily jabbing with my felting needles and BAM- I get stabbed in the leg or finger.
These days I usually use a thick piece of foam because I am forgetful and easily will puncture myself If I am not careful.
In addition to using foam underneath the piece to support it and me, I often have a piece of various sizes inside my work to support it as I shape it. For the felted bowls in this series, you will either need a piece of foam the right size to go inside your bowl as it gets made, or some other soft object that can serve the same purpose. ( I sometimes use a plastic bag filled with old yarn ends)- This will become more clear in the next article.
If you purchase a kit, or foam from a store for felting, it is usually firm but not really hard. This helps keep the needles lasting longer.
I have found my own supports that I like, and have never purchased one.
Basically, any piece of foam large enough to use and thick enough is worth trying out. You may have something lying around the house that can be used. An old foam or fiber pillow with the cover removed, a piece of foam that was shipped with a computer or other object.
My two favorites are an old thick chair seat I cut into blocks, and a yoga block that I no longer use.
Even when I did yoga, I never really used the block, so I didn't mind sacrificing it.
I literally found the big chair cushion on the side of the road. I am a big believer in law of attraction and in fact, have a website about it called flowingmiracles.com in case you want to visit me there.
But anyway, about the foam. I was driving along and I was thinking about a needle felting class I was going to teach at a local yarn shop later in the week. I was wondering what I would use for foam for the students in case there were a lot of them. I noticed this thick seat cushion on the side of the road that had probably fallen off someones truck or car while moving furniture. I stopped, brought it home, cut it up, and washed it. It was pretty filthy.
The blocks are not too pretty where I hacked them with a knife but it works great. It is very thick-about 5 inches.
You don't need to spend money to find foam for felting. Most likely you have something lying around that you can use. Experiment until you find something you like.