The 30-27 designation would indicate that the headset is a JIS standard. (30mm cup sleeve diameter, 27mm crown race diameter) I'd be very surprised if your current headset is not ISO standard so you will not be able to use the headset your LBS sold you, even just the crown race.
Shimano headsets aren't that difficult to source. Another common choice would be a Ritchie or the correct size Tange.
I wouldn't have a lot of confidence in your store.