The garden center has the biggest selection of really good chemicals. Sulfur, Urea, and other soil amendments, root killer also known as copper sulfate. The two main drawbacks with local chemicals are the purity, and amounts available. Generally speaking the chemicals are not reagent grade. They may only have 90% active chemical but frequently that is enough. Sometimes these impurities can be removed by recrystallizing in an appropriate solvent. And, at times, the only available size may be 5 pounds or more.
Another possibility is that some chemicals that are hard to find can be made from common chemicals. For example, iodine is hard to find but can be made from potassium iodide which is available. I generally start with a search for the chemical and then find out what it is used for. A little research can go a long way. But, a good source is to check out the Commonly Available Chemicals list compiled by science educator and consultant, David A. Katz. It was designed for schools that have a limited budget. The PDF is here. Happy Hunting!