Compile a list of keywords to use in Google. Take a minute to write out two and three word phrases that you think would be related to the music site you're looking for. Have at least five but shoot for ten. Input the keywords and perform your searches. Check the websites on the first three pages (normally no one goes beyond page one) and bookmark the ones that are what you need. Repeat for each keyword. This method will get you more than enough.
Visit music blogs and check their blogrolls. A blogroll is a list of sites that a blogger frequents. If the blog is one that covers the topics and genres of music you're interested in, chances are their blogroll has a full list of other similar music sites that cover alike topics. In that same vein, you can read the comments section. Many times readers post a link to other sites that may be of interest to other readers of the blog.
Join a music forum. You can normally find others who are attempting to accomplish the same thing you are or have in the past. By performing a forum search using those keywords you compiled, you can determine whether someone has found what you're looking for already. Otherwise, you can start a new thread and simply ask. Forum members generally love to help and find out information for each other depending on the activity.
Open an account on a social network. Social networks allow you to meet people who have the same interests and may have information you don't have in reference to music sites. There are also music groups within the network you can join. You can even become part of a specialized social network that caters to a certain demographic like music listeners. Networks like Myspace Music, Sonific, Flotones and Last.fm fall into this category.