If you type "DIY color catcher sheets" in the google search bar there are a lot of sites. This is one which describes the general process ...
The color catcher is a piece of white something with soda ash solution dried onto it. You can make your own with soda ash, usually sold in swimming pool supply areas of hardware stores. (Grandma called it washing soda.) Just put a tablespoon of the dry soda into a cup of hot water until it dissolves. Put in a yard of white junk fabric, or ratty toweling, and let it become saturated. Let it dry. Cut off a piece and wash it with your bleedable fabric. The soda ash grabs and holds the dye. You can make a lifetime supply of color catchers with a buck or two of soda ash. For those of you who tie dye, this is exactly the technique for pre-treating fabric before applying dye. Soda plus cotton grabs loose dye molecules.