At a minimum, the defense can subpoena the person to whom the data belongs, who is obligated to provide it. The warrant requirement might prevent them from bypassing the data owner and going to the entity storing the data; if this is the case (I'm not sure it is) and if it is a serious issue in practice, it could be addressed -- nothing in the Constitution limits warrant applications to prosecutors, even if that is the usual practice. So, incorporate in statutes creating warrant requirements shall-issue provisions requiring, on a showing of probable cause that material evidence exists, courts to issue warrants on behalf of the defense (either leaving the defense to execute the warrant, or giving the defense the option of executing the warrant themselves or having bailiffs/marshals execute it on behalf of the court) -- the authority to adopt such a rule applicable to federal courts is obvious, to apply it to state authorities one would invoke the enforcement clause of the 14th Amendment.