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You descend from the Shermans of Yaxley village. Their arms were: Or a lion rampant Sable between three oak leaves Vert.
http://americanheraldry.org/pages/uploads/Notable/sherman.gif
http://www.soysite.com/PhilipShermanCOA.jpg
See: http://books.google.com/books?id=papRAAAAMAAJ&lpg=PA345&ots=8ARWStctl8&dq=%22Henrietta%20Perkins%20Baldwin%22&pg=PA8#v=onepage&q=arms&f=false
There are other arms for other Sherman lineages.
These may not be your arms since they may have differenced them several times through the generations.
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You need to know more about your Sherman ancestors for anyone to do the search. If your lineage is Prussian, then you need to know where in Germany or Poland your ancestors lived so you can contact the regional heraldry society for a lookup. If English, you can contact the College of Arms in London. Either way, there are no surname coats of arms.
Please read the information at this link before searching for or adopting a Sherman coat of arms on the internet. I quote a reply to your general question at length below, which was developed by rec.heraldry:
http://www.heraldica.org/faqs/heraldry.faq
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3: How can I find my coat of arms or my family's coat of arms?
This is a difficult question to answer; it requires a great deal of research and skill.
In most countries in the world, you can bear any arms you want. This is the way in which arms were originally adopted, before codification and regulation by European heralds and rulers.
However, many people consider it wrong to adopt someone else's arms. In some countries, notably Scotland, this is not only dishonourable but illegal.
In particular, there are no laws regulating the use of _non-governmental_ arms in the US. The American government neither grants nor recognizes armory. You can adopt any arms you choose and use them however you want; but you have no particular right to those arms or any other.
If you are descended from someone who was granted arms by some heraldic authority then you may have some claim to those arms within the jurisdiction of that authority. The chances are very good that you do not have any claim on any actual arms. Most people in the world do not.
Exactly what conditions you have to meet to establish such a claim vary considerably from one country to another. At the very least, you will have to prove that a recognized holder of the arms is your ancestor. In some countries, you would have to prove that you are the legal heir of that person. Getting an official recognition of your claim is likely to be expensive and time-consuming; in England, for example, it costs thousands of pounds.
*** Your last name has nothing to do with the matter. ***
Arms are not associated with surnames, but with individuals and, in some countries, with families. The important thing is who your ancestors are, not what surname you happen to bear. The fact that your name happens to be "Smith", for example, gives you no claim whatsoever on any of the thousands of arms borne throughout history by various people named "Smith."
Unfortunately, there are lots of unscrupulous businessmen worldwide who are happy to promulgate false information about the subject of armory. They will happily take your money to tell you "Your Family Arms", which they supply simply by finding an armigerous family that happens to share your surname. We suggest that you avoid these companies; if you want anything more than a decorative wall-hanging, they are a waste of your money. And if you will be happy with any pretty picture to hang on your wall, you can save yourself the trouble of dealing with these companies, and simply choose arms that you like.