If you are a teacher or have a business account you can buy one for $100 here:

https://www.carolina.com/nuclear-energy/cloud-chamber/759240.pr

I have no idea why Carolina restricts the shipping on this. Also, Grocery stores often have dry ice. Just ask a store manager for a bit. Dry ice can be dangerous -- you don't need much.

It also gives an easier to understand description of how to build one.