My wife has often baked my birthday cake and brought it to the restaurant in advance of a birthday dinner. They typically charge a cutting fee. They seemed very happy to have our business.
The important thing to note is to call and discuss this with the business in advance. If you want to supply your own parts, just don't show up with them in hand. Call and speak to the manager and explain why you'd like to use your parts. If it's a question of cost, maybe they can discount their parts. If it's some other reason such as a specialty part they don't carry or you want to rotate out your parts, they will surely understand. Communications is the key.