I'm sorry the dealer doesn't know what they are doing.
If you can't get a stock replacement, your only other options are:
1) Go to a Junk yard and pull a spring from a wrecked convertible stang (or all four from a hard top)
2) Go aftermarket and buy a complete set of 4 lowering springs. I think the smallest (least amount) of drop comes from the FRPP "B" springs. Someone feel free to correct me on that one though.