My experience has been that if you act professional and businesslike in your dealings with these people they will treat you with coutesy and respect. You should not lie or fabricate a story you think they want to hear. They have exceptionally accurate BS meters. If you are a student, then say so. If you bought a starter kit, or a development board, tell them that. If one chip from the starter kit is not enough to support your continued development activities while you distribute prototypes for testing and evaluation tell them that.
In the US, small quantities of parts are normally sold through distributors, like Arrow, and Future, and Schuster. It would not hurt to establish relationships with these companies since distributors have a very easy time getting samples. It also demonstrates your willingness to pay for parts if your development activities prove fruitful. Ask if you can establish an account and get Net 30 terms. If you can do this, it will demonstrate a professional and businesslike approach to the procurement of material.
If you communicate by phone make sure that you speak clearly and directly. These are busy people -- don't waste their time. If you communicate in writing avoid slang expressions and instant messaging shorthand. Use the spell checker and grammar checker. There is absolutely nothing that creates a worse impression than a letter that looks like it was written by a child unskilled in the use of his own language.