The way you get a sponsor form (rec) is by asking someone to do one for you and sending her your resume. So it wouuld seem you already know her address. And yes, you would send a thank you.
i'm guessing that you might have one contact for recs. who has friends in other sororities that she is going to send your information to. your rec.contact should not mind(and most likely will be impressed) if you ask her for the addresses of the women who wrote recs. for you, so that you can write them a thank you.
__________________ I live in Fantasyland and I have waterfront property.