Coin dealer, pawn shop, eBay.
Currently a 1880 S Morgan silver dollar in MS65 uncirculated condition is worth $156.
1880-series US $2 bills were issued as United States Notes rather than silver certificates. Please see the question "What is the value of an 1880 US 2 dollar bill?" for more information.
Currently as of April 2012 a 1880 Morgan silver dollar in MS63 uncirculated condition is worth around 63 dollars.
Take it to a US currency collector.
Responses to questions on Answers.com are normally specific to one item. Please check for questions "What is the value of an 18xx US silver dollar?", where 18xx is 1880, 1881, etc.
Simply take it to your local coin shop or sell it online.
A silver dollar with the national motto E PLURIBUS UNUM on it dated 1880 is actually called a MORGAN dollar, most all US coins have this motto somewhere on the coin. It's a very commoncoin with a retail value of $36.00-$42.00
Your coin is called a Morgan dollar. "American Eagle" coins are modern bullion pieces made since 1986 for collectors and investors; Morgan, Peace, and other standard-issue silver dollars were made for use in ordinary commerce. They were discontinued in 1935. Please see the question "What is the value of an 1880 US silver dollar?" for more information.
There is 24 grams of silver in a 1891 US silver dollar.
Higher-value silver certificates weren't printed very often. The only series dates for $100 silver certificates were 1878, 1880, and 1891.
As of 01/2010, these coins sell for about $15.00
You can take them to a local coin, collectible, or pawn shop and sell them to them. You can also sell the on an online auction site such as eBay.