Network, network, network.
Really... the time to not be looking is when you need one. This is an all year, every day event for a church. If you have a regular worship leader/pastor you should be engaging them to help in this area. Be sensitive here, telling your current worship leader you would like them to help find people that can lead on occasion may be akin to asking your girlfriend/boyfriend if she/he has any single friends.
A typical Craigslist ad might read,
“Looking for a guest worship leader to play from time to time at our church. Service typically includes contemporary and modern worship songs with some hymns. Solo or bands welcome to inquire”.
When you post the ad it will default to anonymous posting which means people will not see your name unless you put it in the posting. (Not recommended). When they reply, send them the church website with a brief explanation of what you are looking for. ASK THEM what they would charge. If it is reasonable, contact them and ask for YouTube links to services they have played and or for references from the last 2-3 churches they have played.
(Refer to the “How much should I pay” section on determining pay for services)