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 shipping 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.
Stores That Sell Copper Sulfate
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). This product contains solid granules of copper sulfate. It's very easy to work with. If you wish to use it to grow crystals, dissolve it in hot water and then use a coffee filter to remove large impurities.
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.
It's fairly easy to find copper sulfate solutions, but you'll probably need to evaporate off some water to get the concentration you need.