The Music Library has over 40,000 CDs and 15,000 LPs in its stacks, and between Naxos, Music Online, Amazon, Google, and AllMusic, there are literally countless places to find inspiration. Survey the covers of your favorite albums, featuring your favorite artists, and make notes about what you like and don't like.
There are many places you can find public domain or Creative Commons licensed images for your recording project's cover art. Here are a few of my favorite image sources. Be sure to provide attribution for CC licensed images and to follow any other license requirements (no derivatives, share alike, non-commercial) and to provide a citation for the source of the image(s) you use.