Yomomma: According to NOE, the BX-410 (Ballistics Products) wad, is recommended. Can order these, through Wil Bilozir. Another choice: Gualandi BX-410. Sporteque, or possibly Hummason, may be sources.
Since the BX-410 is essentially a flat based wad (with a slight lip to contain the base of the slug)...I suppose you could also simply cut the petals off an ordinary 410 wad....CB 5050-410HS, for example.... then adjust stack height with fiber or cork wads/cards etc., to get the right crimp height.
Al