hidden code

Hi, the issue here is not malicious code but some code I must have put myself, somewhere, and can’t delete…
I used to use a certain provider for my newsletter and had sign in forms and sign in links in few places in my blog, I have tried to delete them all (i.e. replace with new sign in form for the new provider) but there are still some out there, people keep signing in and I don’t know where from!

Assume the provider I was using was called NEWSLETTER.COM how do I find all occurences of newsletter.com?

I have already tried the obvious: search in posts, comments and pages, sidebar, footer and header.

But some are already out there…

Thanks for your help, really appreciated!

Letizia