이모님 and 아가씨 are commonly used in small places, especially the traditional mom-and-pop diners.
They sound out of place in a modern store, though, since they are non-professional terms(이모님 = aunt, 아가씨 = miss, young lady).
There are other terms like 점원(clerk), 사환(pager, errand boy) and 계산원(cashier), but these are very dry sounding and unpopular.
A newer and recently popularized word is 도우미 (coined from 도움 - help), which can be translated as "helper".
I think this can be used in any kind of professional settings.
It seems not as commonly used as a direct address word yet, but I feel it is on its way there.
So at a small, family-run place, you may call them 아가씨(young female), 아저씨(young or older male), 이모님(older female), and 사장님(the owner). At a larger or more professional place, you can just use 도우미님 for anyone working there regardless of sex or age.