You can use "이모님" to elder female clerks (older than late 40s), in special, in a restaurant. You can use "아가씨" to young female clerks, both of restaurants and shops. Both of words is commonly using to address a female clerk, however, it is depending on the age and the situation (what kind of store you're in). Therefore, I also recommend you to use "여기요" to address you need their help. (me, neither, I rarely use "이모님, 아가씨"). FYI. Your Korean spelling is perfect!

you can call them 이모님 but you don't need to call them directly. I think you'd better to appeal where you are so they can notice you! so! my bottom line is "여기요" or "저기요" If you need help wherever you go or what situation you are in you can say "여기요" or "저기요" It's polite way to call them.

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.