Copper sulfate pentahydrate (also known as copper sulphate pentahydrate or simply copper sulfate or copper sulphate) is the chemical used to grow bright blue copper sulfate crystals
. It's easy to find online, but there are shopping restrictions to some locations, plus the weight of the chemical typically makes shipping comparable to the price of the chemical. Fortunately, copper sulfate is a useful chemical that you may be able to find in a store... if you know where to look.
I found copper sulfate at Home Depot as Zep Root Killer, which is labeled as copper sulfate pentahydrate and is sold with septic tank and drain cleaning chemicals (not with other garden root killers or with other plumbing chemicals). Less commonly, you can find copper sulfate granules sold as an algicide for ponds. Ask for an algicide solid or powder and make certain the product is labeled as copper sulfate or copper sulfate pentahydrate.