When you enter the store/start a conversation with the clerk:

What a clerk might say:

May I help you?

Can I help you?

Can I help you find something?

What can I do for you?

 

What a customer might say:

Excuse me. Do you work here?

(Can you help me?)

Can I ask you something?
 

What a customer might respond:
Yes. I'm looking for ________________. 
Do you have any ___________ (s)?

Can you tell me where the ___________is/are?

 

When you check out/leave the store: 
Clerk:

Did you find everything you needed? 
Did you find what you were looking for?

Did you find everything okay?

Will that be all (for today)?

Is that everything?

(Will there be) anything else?

 

Customer: 
That's all for today. 
That's it. Thanks.