How to easily find or create parts for Eagle schematic/board layout
(1) Finding existing Eagle parts already created by other people out there: I recommend the following four sources ( aside from Googling "partname Eagle" ;-) ):


You can look in Cadsoft's searchable collection of Eagle libraries
You can use Jason Tribbeck's Eagle part/package/device search, which allows wildcard/partial-term searches
You can use Esawdust's Eagle part search which also allows wildcards and regular expressions in search terms.
You can get the library that Sparkfun mains; it's a huge collection of common parts called Sparkfun.lbr, which is located at this page.


A WORD OF CAUTION (courtesy of user @Grant)... When using others' libraries or parts, first compare it to the datasheet, and/or print it out on paper for comparison to actual part. There are some untested and/or incorrectly dimensions footprints out there. 

(2) Creating your own parts:
It is not that hard at all to make Eagle parts for most things; frankly, if you are able to construct a schematic and a layout, making parts yourself will be hardly a step beyond. I have four pointers:


For learning part creation, I suggest you start with these three tutorials; the creator spent the effort to make them very beginner-friendly: Tutorial #12, Tutorial #13, and Tutorial #14 on this Eagle tutorial-page.
Start learning with simple examples such as a resistor, a DIP part, or even an SOIC-8 part to understand how it works; the clarity of understanding will then readily carry over to more complex parts.
If the part has a footprint that is a common one (such as SOIC-8), just copy an existing part's footprint.
Follow the manufacturer-recommended layout: Nearly all parts' datasheets prescribe dimensions for a recommended footprints/layout for the part; if you follow those precisely, life will be easier and you'll have a part ready in no time.