Search on the AS or A Level Full Course Pages
The videos are organised in the same order as the specification, so you can use that as a guide to find any topic you need.
Instructions
From the subject home page choose either the "All AS Tutorials" or "All A Level Tutorials" options from the Learn section.
This will give you a full list of every lesson
Search on this page for the key words that relate to the lesson you are looking for.
PC/Windows: Control+F
Mac: Command+F