The weight lifting buckets that come with KidWind Advanced Wind Experiment Kit (KW-AWX) and KidWind Basic Wind Experiment Kit (KW-BWX) and their respective classroom packs can be purchased from IKEA.
The buckets are the KALAS Tumbler.

You can either purchase them in store or online. The link to the product page is:
http://www.ikea.com/us/en/catalog/products/10192956/