I wish API water test kits came with plastic tubes because my clumsy ass keeps breaking them...I'm down to one tube for 4 tests...checking water takes too long now lol...sorry just thought I'd share the frustration...
Phone them, tell them how you are a "clumsy ass" and I'm sure they'd sell you some spares?
You could probably get test tubes from your chemists, but not sure they'll do plastic at the risk it would react with whatever you were testing
In my case, I learned to always, even when testing, stand the tubes in the tray they came in. Keeps them from falling over and breaking. Hope this helps.