i got it at the pond outlet. Exactly the site you have identified. I found the same thing you did, that the prices varied wildly. It was a few dollars cheaper then, and the shipping was free at the time. It's great stuff, as it weighs nothing. You just pull the bag outta the barrel and give it a quick shake and all the crud just falls right off. Boom. Pop it back in the barrel and I'm done. I'd like to see someone with a bagful of soaking wet sponges try that easily. I just wanted something light, and something that would last forever. Personally, it was the best choice for me. I didn't want to think about replacing anything in a few years (which is also why I went with the matala mats). I got a green and black one--the other colors are just too fine for a barrel in my opinion. Of course, I don't disparage anyone with the sponges. They are a great choice if that's what you want.
PL roofing sealant is safe in water, yes. It's a ponding standby that many pros use--which is how I came to use it. I've used it in a host of places in my pond, and I've never had a problem. I know people have called the manufacturer about it too. It's thicker than the black aquarium sealant, which is what I love about it. And if you ever screw anything up, it peels right off easily.
I put a conduit connection in my barrel that I just could not get to stop leaking, I had not tightened it enough apparently. Thankfully, I was able to just pull off the PL, tighten the pipes and then reapply. Poof! Leak stopped. Had I used something else, I might have had to cut the pipe or worse--get rid of a perfectly good barrel.
