What companies to offer my services to?

I've reached out to a few of you regarding a start up company my sister in law and I have begun, and to those of you that have given me advice I am very grateful. That being said, I'm wondering if anyone on this board can give me advice as to what types of companies we should be prospecting.

We've started a handwritten direct mail marketing company. The bottom line is that we'll hand-write the mailing and return address on envelopes for $0.25 per piece, and stuff marketing materials for an additional $0.10. If not provided, we will purchase the envelopes and stamps for these companies at cost, offering them the receipts for reimbursement. I've generated our mailers this evening, breaking down the benefits of handwritten direct marketing campaigns along with a post it stuck to the letter that states "You opening this letter is proof that handwritten direct mail marketing works!". I think I have a solid product, we offer a very fair price, now the question is who to mail the letters to?

I was thinking of Doctor's offices, Lawyer's, family owned restaurant owners, mechanics, etc. I think this is best targeted towards small business or family owned companies, but where to find them? Another thought I had was to start collecting the 'junk' mail I receive at my home from companies that send pre-printed envelopes and use those as leads. Does anyone on this board have ideas of what types of companies to target that would be a decent turnaround so I can hit up 411.com or yellowpages.com and start searching?

Your input and advice is much appreciated!