Extra Pieces for Lego Gadgets Book
Rebrickable has inventory of elements included with this book. This website has also an options to add these elements to wanted list for Bricklink as well as BrickOw. Each element has links to both websites (Bricklink and BrickOwl) so use the one you prefer to acquire elements you actually need.
Once you know the element you need you could also buy them directly from LEGO via &quot;Bricks &amp; Pieces&quot; found on LEGO Support page. Although it is a bit slightly more complicated since book doesn't have a set number and you have to look up each element individually.