You can purchase ingredients from the Public catalogue by
choosing the ingredient you want and then click on Get this Item. The
purchased ingredient will be added to your cart where you can click on checkout
to finalize the purchasing process.
Go to Public Catalogue choose a layer then click
Add to cart > click on checkout in your cart list.
Please note that the user performing the Add to cart process has to be an explorer while the user performing the checkout has to be a purchaser.