I was able to go to a local plastic shop with a jar and buy a small amount for a project I was working on. Later I found another plastic shop who was able to sell me a pint.
Pretty easy to find.
Regarding safety, this is a solvent and will evaporate completely. This product does not glue or cement the pieces together, it melts them together. By the time it has done it's job it will be long gone.
Weldon3 has been used in acrylic aquaria for extremely delicate marine invertabrates. That's good enough for me. I also know of a guy who builds terraria exclusively for dart frogs. He used the same product with no apparent problems.