Simply use FindControl method.
Here is one place you can find "find control":
http://odetocode.com/Articles/116.aspx
one example:
public void YourRepeater_ItemCommand(object s, RepeaterCommandEventArgs e)
{
UserControl _userctrl = (UserControl)e.Item.FindControl("UserControlname");
if(_userctrl != null)
{
//Do your job here
}
}
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For more troubleshooting refer this articles/threads or you have different scenario try this out:
http://www.velocityreviews.com/forums/t90329-find-control-inside-a-repeater.html
http://www.eggheadcafe.com/conversation.aspx?messageid=32149707&threadid=32149707
http://forums.asp.net/t/1110178.aspx
http://www.developerbarn.com/net-development/643-reference-user-control-nested-repeater.html