You can buy a piecepack from the following vendors. Other commercial piecepacks are out of print. But you might find older used versions or non-commercial versions elsewhere.
The Workman edition (“The Infinite Board Game”) is available worldwide in various bookshops and online shops, including Amazon. It is made out of durable plastic and includes a rulebook with 50 games.
There are different piecepacks on The Game Crafter. They are mostly made out of cardboard and are printed on demand. They ship worldwide from the USA.
Because the design of the piecepack is in the Public Domain, you can create your own and even sell it. Some people have shared their designs so you can print them at home. Or create yours completely from scratch.
This is one example of a self-made piecepack with a downloadable design.