I bought a few shifter cables and some were missing the compression spring. While at The Home Depot I stumbled across a pack of compression springs. The 3/8" one in the pack WILL work to replace your missing or damaged one. It is only 1/16" larger in diameter but will fit the shifter and cable. It is 1/4" longer but can be cut to fit.
Manufactured by EVERBILT, SKU# 685 436
Manufactured by EVERBILT, SKU# 685 436