I'd suggest going somewhere that rents machines. I think almost all of them have defoamer. You may pay double to triple what it's worth though. Rental chems are usually VERY high dollar. I've heard the thing about fabric softener too, but never tried it. I believe years ago I even used regular table salt to diffuse foam in my machine on a few occasions and it seemed to work ok. That might not work in a heavy foam situation though.
When using my portable. I put some in the recovery thank to prevent foam being sucked into the vacuum motors. My wife went to buy me some from a Janitorial supplies store that was open Saturday when I needed it. It was liquid, it worked but I think I like the powder better as many others here do as well.