The Sunstar countershaft sprocket P/N you want is 51115 - if you want the OE 15 teeth.
If you want to drop a tooth, 51114.
If you want to add a tooth, 51116.
(Most bikes that go +1 need a little clearance grinding to the countershaft cover).
Sunstar's MSRP for all of these is $22.71
I don't know why your dealer couldn't find this info, it's easily found on the
Sunstar site by looking up a Suzuki GSF1200S...