Note that under 18 U.S. Code § 2704, a related problem is that only governmental entities can require preservation of data.
I have not seen any legal authority for the defense getting a search warrant. If you can find any authority or precedent for that, I would love to see it. It's an awesome idea.
I wouldn't expect so, since I proposed creating statutory authority for them as a solution to the problem you described that would seem to be well within the parameters of the Constitution, so that we don't have to sacrifice protection against warrantless searches and seizures by government in order to preserve the rights of defendants to secure evidence for their defense.
I'm quite aware that defense-initiated warrants are not an existing thing.
Doesn't that count as obstruction of justice?
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.