Ferrites are clip-on magnets to prevent interference pickup in cables.
You’ll need two for the USB cable for your frequency generator, two for the cable connecting the generator to the PA3 and it does not hurt to put two on the power cable from the large power supply (from step 4) to the PA3 as well. Six in total.
If your USB cable for your frequency generator already has ferrites on it, you should not need to add two more.
These from Farnell/Newark will do. Farnell/Newark part # 03H8829. They may be found cheap on eBay or dx.com as well.