I've seen 3/8" offered here on a quick search https://www.usplastic.com/catalog/item.aspx?itemid=23369&catid=489
They have a quite a few model air check valves, might want to peruse through them. Double check if they require mounting in a certain direction and the lowest cracking pressure available.
I mounted my central air pump higher than the tanks so don't need one but you will definitely need one.
Which model Alita do you have?
Good lookin out thanks! This one is the AP40.