Ed,
Unless someone here or in the Yahoo A/C Group is parting out a QCGB, your best bet is eBay. If neither gear is mentioned, look for sellers with several Atlas lathe parts for sale. Write them and ask, quoting both part number and description. If they have a complete GB I doubt that they would sell part of it. But they could have a parts unit that they aren't bothering to list parts from.
You could also probably buy stock Boston gears from a number of places but they will require modification. I forget what the DP and face width is for all of the gears in the GB but if the two that you need are 16 DP, Mcmaster's prices are in the mid $40's per gear. And broaches are in the high $50's, plus bushing and shims maybe $20. So unless you were doing several of one size, more expensive than Clausing's prices. Used is likely to be your cheapest route.
