I got my new balancer from rockauto.com for about $65 +s&h.
Try looking up a Chev, 1975, P30 Van, 4.8l, engine, balancer
Where is the timing tab on your cover? Driver's side of center or passenger side? You can establish a new one at TDC if you need to but if your outer ring is slipping on old, dryed out rubber then it won't help for very long!
