Unless you're dead set on buying a name brand one, there are other options.
Packaging foam suppliers (which is what I do) supply Polyethylene (PE) foam extrusions in different diameters, and you can find them online pretty cheap.
My buddy owns a commercial caulking business, and he uses a ton of different size "Backer Rod" to fill gaps, which is essentially a piece of PE. I grabbed an extra piece of 6" diameter from him, and it works great. Best of all, you can buy the stuff cheap and any home improvement store. Just get a heavier density foam so it holds up.
I'm all about being cost-efficient! You should see my home gym.
