if it is not fast flight string for sure .500 spine - if it is fast flight string - depending on how heavy you want the arrows you could use a .400 spine with a heavier tip. The tip weight will be dependent on the length and spine of the arrow and how flies.
You can buy arrows one at a time from lancaster archery supply - I would buy one of each .500 and .400 and then I would buy some differnet weight field points for testing - say some 100, 125, 145, 175 grain tips and then play around till you get the right combo that bareshafts good out of your bow.