Peter -
The best sailing answer would be to buy a new 155, but that may not be the best economic answer. In general, most of the used sails I have seen are good sails but not necessarily good racing sails. They are most likely made of dacron and of a heavier fabric weight then you would want for light air conditions. I think it is easier to buy a used spinnaker to race with than a used headsail.
The best two places I have found for buying/selling used sails are Minney's in Newport Beach (
http://www.minneysyachtsurplus.com/ ) and Bacon Sails in Annapolis (
http://www.baconsails.com/sailsearch ). Minney's has a huge selection of sails that are searchable online by luff size. Most of their sails are listed in "good" condition with some listed as "excellent" and others as "fair". Last year I sold two Catalina 34T mains to Minney's that I never used, an original UK main (1986) that was in excellent condition, and a racing main that was in fair condition. Likewise, Bacon Sails has an online search capability with a large inventory. I bought a windseeker/Dazy staysail in excellent condition from Bacon last year for a very reasonable price, including shipping.
Your best bet would be to measure your existing sail and/or to measure your hoist as there can be a variance from one boat to another on depending how the roller fuller was set up. For example, my hoist in 10" longer than the boat next to me because of where they set the top of the fuller. Regardless, having a sail cut down a little bit to fit your boat is a simple task for a sailmaker and is not very expensive.
Regarding the Pacific Cup, most of the Cruiser class boats do use spinnakers while those that don't are given a non-spinnaker offset of 16 to 18 seconds per mile to their PHRF rating.
Gary