We found 2 Reddit comments about Rit 15 1 Oz Black Rit Powder Dye. Here are the top ones, ranked by their Reddit score.
Available at a lot of stores.
I hope someone has pointed out that those slacks can be salvaged. Go to a store like Target, WalMart or any craft/hobby store that has a fabric or laundry section. In that section there will be an all purpose dye/tint. Rit is a common brand that costs $2-6, and is easy to do. It isn't recommended for 100% polyester, 100% acrylic or anything that's dry clean only, but it's worth a shot before replacing slacks, anyway. Not like they can be more ruined than they are.