The best replacement for copper sulfate is copper chloride.

The best alternatives to copper sulfate are rated as following:
★★★★★  copper chloride
★★★★  copper gluconate
★★★  rock salt
★★★  cupric sulfate
★★  foaming root killer
★★  cutrine
★  dichlobenil
★  copper hydroxide
★  rootx

Your alternative for copper sulfate is not listed? Suggest!
Speaking about copper sulfate, one probably shouldn't forget about zinc.

Other search terms related to copper sulfate and their ratings are:
★★★★★ zinc
★★★★★ ammonia
★★★★★ sodium hydroxide
★★★ aluminum
★★ iron
★★ potassium iodide
★★ iron reaction
★ iron nail
★ sodium carbonate
steel wool
ammonium
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Google Trends

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copper sulfate
copper chloride
copper gluconate