Yes there is plenty of legal pay for sheet music and legal free sheet music available on the internet. Also there is plenty of illegal free sheet music and illegal pay for of sheet music available but we won’t speak further of that.
Most sheet music dot com sources will show you page one of a song with a big water mark on it. So you can easily get a peek at the score and see if it matches your playing skill level. Two examples are www.musicnotes.com and www.sheetmusicplus.com .
Yamaha Music Soft , www.yamahamusicsoft.com offers a special IPAD app where you can conveniently purchase and play music scores right from the app. Even greater is you can turn on and off the different music instrument and vocal tracks allowing you to play and or sing along as if you were in a group setting.
A music composition older than 70 years is beyond copyright protection period in the USA and said to be in the Public Domain. There are several online sites offering access to the old classics and one of my favorites is the International Music Score Library Project at www.imslp.org . There are also university libraries that have online database of music here is one example: http://library.indstate.edu/about/units/rbsc/kirk/sheet.html#All
You can find this beautiful score of America the Beautiful there.
Another resource I’ve found can be handy is images.google.com. For example on the images.google.com web page search for “song: mary hand a little lamb” or “lyrics: mary had a little lamb”. (don’t enter the quote characters)
There are many online sites you can visit to purchase sheet music for songs and even complete artist songbooks like Amazon and even EBAY. If you want to go exploring online to the other side of the world then in www.google.com search for “sheet music site: *.ru” that would bring of a list of sites in Russia.
Last I’ll mention another inexpensive sheet music source and that is second hand book stores. Look for old hymn books which can have hundreds of scores in them and often can be purchased for just a dollar.