I have had good luck frequenting flea markets and junk shops. Look for old "T" augers with the bits in good shape. The handle can be cut off and the shank turned by a machinist to fit your chuck. I seldom pay more than $10 for a bit if I'm patient. The machining may cost an additional $10-$15.

One inch and 1 1/2 inch bits are also available from Irwin. Costs are not too bad. It's the 2" bits that cost something like $140 new.