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Amazon [amazon.com] has 8-Ounce Irwin Tools Strait-Line High-Visibility Marking Chalk (64907) for $ 1.59. Shipping is free w/ Prime or on $ 25+ or $ 35+ orders (minimum requirement varies by location).
Product Description from Store[list][*]High-visibility chalk is Ideal for low light conditions and on dark surfaces.[*]Durable formula is weather-resistant and suitable for interior or exterior use.