It's a 1/4 inch type of tubing, used typically by plumbers, but it's probably a multi-use thing (although God knows it was never meant to be used on children). I don't think that it's called "plumbing line" officially, but I think most people who run to Lowes to buy it can ask for that and usually get aimed in the direction of the PVC tubing.
Be careful with reading. I know I've said this a lot recently, but when I started reading what they actually write (shortly after Gianna was born), it messed me up a lot, emotionally. Like, it affected my marriage, the way I interacted with a family member who seems to like the Pearls, etc. It was BAD. I wanted to get a primary source for my arguments against them, and it really shook me to the core- the Pearls are, I firmly believe, being used as a tool of Satan himself. My family member in law keeps trying to get DH to agree to take a copy of the book to at least look at it and decide for himself, but I've set the firm boundary that if the book ever shows up in my house at all for any reason, it's going in the fireplace with a hefty soaking of fuel and as many matches that I can throw at it. It's pure evil. I know another mom on these boards gave the book a shot a short while ago, and she had a similar visceral reaction. Make sure your faith is rock-solid, make sure you are surrounding yourself with people who can re-orient you spiritually and emotionally, and know when to put it down, whether you borrow it or read it or somehow come into contact with it somehow. I read it while it was still online, and that was still bad enough.