Please all, do check the difference between a certificate and a certification. At present, the only legal nurse certification approved by the American Board of Nursing Specialties is the LNCC, Legal Nurse Consultant, Certified. Vickie Milazzo and many others run "legal nurse cert programs" (with dismal post-course employment rates...but hey, she's got your $$$ so SHE's doing well) but no others have passed the rigorous process to have their examination approved by the ABNS. The LNCC Cert board is an affiliate of the American Association of Legal Nurse Consultants, AALNC, a nonprofit (unlike the others), which see.
That said, almost any LNC will tell you that an attorney probaboy doesn't give a dang about the letters after your name if you know how to do legal nurse work. The AALNC, in my opinion, does the best job on preparing you for that, too. Their core curriculum, the two-volume Legal Nurse Consulting Practices, 3rd ed., AM Peterson and L Kopishke, eds., is a really good place to start. (Disclaimer: No financial connection to the book or its publisher, other than I bought one and it's on my desk right now.) They also have webinars, conferences, and affiliated chapters all over the country. Get hooked up with them and start there.