You can find royalty free music at places like http://istockphoto.com - prices are around 25 credits (dollars) a clip.
If you are looking for more traditional album / track sources, there are sites like http://www.royaltyfreemusic.com/ where you can purchase a track (or a subscription for multiple tracks) with the non-recurring royalty fees included. It's not 99 cents (single tracks are $60, albums are $120, subscriptions are $300) but it does address the issue.
To the specifics of your question - can I use a portion of commercial music as an intro on my commercial podcast - fair use cases are subject to interpretation (your results may vary). 30 seconds or less seems to be the common chatter about what is acceptable fair use. To determine whether you want to go that way, I suggest looking at http://fairuse.stanford.edu and determine your risk aversion level.
A relevant music case example:
Fair use. A television film crew, covering an Italian festival in Manhattan, recorded a band playing a portion of a copyrighted song "Dove sta Zaza." The music was replayed during a news broadcast. Important factors: Only a portion of the song was used, it was incidental to the news event and did not result in any actual damage to the composer or to the market for the work. ( Italian Book Corp, v. American Broadcasting Co., 458 F. Supp. 65 (S.D. N.Y. 1978).)
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.