Hi, Kimmy. There are so many different sources! I usually make my own and I'm pretty sure most others do too. I know some who occasionally use scanned in paper and I know a lot of the programs come with pre-made backgrounds too. There are lots of sources on the internet to download background tiles! In fact, here's a link that was posted as the Hot Link of the week on the homepage here at SBB a while back:
http://www.eosdev.com/eosdev_Backgrounds.htm. I'm sure there are many other great sites that others will share with you
[edited to fix the link]