Here is a link i found for the replacement battery - but have NOT used.
http://www.akku-wechsel.de/shop/alternativer-akku-fur-garmin-forerunner-405-300mah-3-7v-pd3048-lir3048.html
Two other useful links for dismantling and replacing the battery are:
http://www.jimandkatrin.com/CodeBlog/post/Forerunner-305-Battery-Replacement.aspx
http://sites.google.com/site/ctorunner/dismantleforerunner205
Hope it helps
GONZO