Hey...
I usually NEVER endorse any marine parts suppliers, but have great luck with the following:
Go2marine:
(Here's a coil for under $30, Thuderbolt for Merc, just an example)
http://www.go2marine.com/product/834...l-18-5438.html
Also have used:
Marine Engine:
http://www.marineengine.com/
Marine Parts Plus:
http://www.marinepartsplus.com/aboutus.html
All with very competitive prices, good customer service, and fast delivery.
Good luck !
Bigdog