If you look in a catalog, their should be a headset training thing. You put it on your horse and it holds their head in a high, medium, or low headset.
PLEASE do not use gadgets to get the headset you need. A correct headset comes from impulsion, flexion, suppleness and collection. I.e. Training, time and patience.
Do you have a trainer? I have no experience with the WP headset, so can't help you there. But you should have established transitions, steering, lateral flexion, forward and impulsion, and a good stop BEFORE you start asking for a headset.