The part you are referring to is called the manifold cap and can be purchased as a set of three or individually (please note each filter has a dedicated cap). I have provided the links below for your convenience:
https://www.wahlwater.com/Canature-475-Pro-RO-Manifold-Cap-Set-of-3-WW043_p_1341.html
https://www.wahlwater.com/Canature-475-Pro-RO-Manifold-Cap-Pre-Sediment-70030058_p_1338.html
https://www.wahlwater.com/Canature-475-Pro-RO-Manifold-Cap-Pre-Carbon-70030059_p_1339.html
https://www.wahlwater.com/Canature-475-Pro-RO-Manifold-Cap-Post-Carbon-70030060_p_1340.html