The Streamkeepers Catalog
Not an exhaustive supply but some handy items, e.g., nets and thermometers.

Lamotte
This company markets water testing kits specifically for educators.

Connecticut Department of Environmental Protection
The CT DEP has a user-friendly protocol for sampling and identifying benthic macroinvertebrates as a way of determining water quality in a “wade-able” stream. It is designed for volunteer groups in CT but could be adapted for local classroom use. Their website provides down-loadable identification cards and instructions, including a detailed equipment list. The equipment itself can be assembled from household items, plus a scientific supply company or one of the sources above.
[If you are interested in sampling benthic macroinvertebrates, there is MUCH more information available, but this is a good manageable starting point.]