You can buy just the inserts, you know with the 1/4" hex on one end and the clutch on the other.
I bought a few at the local hardware store.
you can also get the bits from McMaster Carr. http://www.mcmaster.com/
page 2755 in the online catalog, all 5 sizes for less than $6 total.
part numbers for the short ones:
longer ones available, look at the page mentioned above.
Should be fine in the impact, and if not then they were cheap...