Engish expressions form making an order

I've been usually using the following expressions "can I have a small coffee, I wanna get a small coffee, a small coffee please" for the years i lived in the states. I guess thats why my english speaking ability never quite imporves. native speakers, could you tell me the expressions that u use to make an order that naturally come outta your mouth

for example, "I wanna grab that muffin, I need.... "

also, the usual expression that I use to find a product that I want, lets say in Best buy or Barnes and nobles is the following;"excuse me could u help me finding something. I am looking for..." I am just tired of saying this expression over and over. I need other natural expressions that native speakers usually use for this too.

Thank you!