The cheapest well functioning birdshot that can be found for SG training. Example 25 rds for $4 - 5. For use on Steel plates, clays, etc.
A bunch of cheap buckshot ( Ex. 5 rds for $3)would be next for training in limited amounts due to cost.
Finally, shoot some good quality / Real World Use Buck or Slug for confirmation of patterns, usually a "Tactical or Low recoil" load as made by Federal, Remington, or Winchester.
SG involves lots of hands on Gun training, Reloading all the time, lots of Reps. So Cheap Birdshot is the key.