Soda crystals, also called washing soda, is the (somewhat) common name for sodium carbonate decahydrate. It is supposedly available as a household cleaning agent (look in the cleaning aisle of your supermarket), perhaps slightly more available in Britain (although we are unaware of any brand names it may be sold under).

Arm & Hammer sells a clumpy baking soda product in a shaker, which it refers to as crystals, but which is not the same thing. But if you fail in your search for soda crystals, go ahead and use baking soda, which is pretty similar. And don’t forget to send a lovely thank-you note to the gift giver!