Are you referring to the threaded adjusting bushings in the end of the front spring that work off the A-arms? ... or the center pad on the spring for mounting to the cross-member?
GM used to only sell the whole spring with adjusting bushings.

Don't know if just the bushings are available aftermarket now.
Occasionally, someone may offer just the bushings for sale.
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^VIR Turn 3 '08^-MungoZ06-'03 QS/ModRed - NCCC, NASA NCM Member - Z06 Fest IV,V,VI,VII