How can I include house address finder in my application?

Question

I am building EPOS system (takeaway order software) with C# and everything went as I planed and I almost done programming it.

The application works where customers provide their Postcode when they want to order something and based on their postcode I should get a list of street names and when I select the street name I get a list of house numbers, then select the number to deliver the food to.

I noticed that all nowadays takeaways use this address finder which included in their EPOS which works with no need for internet connection.

Now the problem is: know do I build / include this address finder in my EPOS system? I do not want to use online address search in my EPOS. Do I have to use another special application to like address finder?

I am really stuck at this point and I can not go further programming my POS till I solve this address finder issue. Please help me with any peace of information you know in how to solve this issue. Thank you