Check your local music stores. Piano stores typically stock instructional books for teachers and students. Sheet music and books of popular songs, religious, classics, country, kids and other themes are available on various levels of proficiency. Music stores that specialize in guitars and/or band instruments typically carry piano sheet music for solos, duets, ensembles and accompaniment for vocal or instrumental solos.
Shop your local bookstores. Large chain bookstores have a section for songbooks. Specialty themed books and "fake" books (focused primarily on basic melody and chords) are particularly popular. You may find piano sheet music and books in the Instruments and Instructional section as well.
Search ebay.com and other online stores such as sheetmusicplus.com for piano sheet music. Prices and availability may vary on auction sites, but chances of finding older or out-of-print piano sheet music can be high. Online retail stores for piano sheet music may offer large stock and options than local vendors.
Purchase and download custom piano sheet music at print-on-demand web sites such as musicnotes.com. Sample previews, genre search and difficulty levels enable custom options. Some sheet music download sites even allow the customer to transpose or specify the desired key.