Marie-
Go out to the company's website. Sometimes they list key employees and you might find her last name there.
If not, do a Google or Yahoo search with the following key words: manager's first name AND company name, as sometimes a manager is quoted in an article or their promotion is announced in the newspaper. These events are often published on the web and you'll find her full name listed in the article or announcement.
Lastly, if these 2 things don't work, just call into the company and tell them you are mailing a letter to the manager and you want to verify the address and the correct spelling of the manager's name. Tell them the street address and ask them if it's correct. Then say:
"Ok...I'm sending this to the attention of Michele...and what is the correct spelling of her last name?" They'll immediately give it to you.
Hope this helps. I have several thank you letter examples if you need help putting one together. See nav bar on left.