Could they get a warrant for that data anyway?

Not if you don't have the data. This is one of the reasons that google changed how it tracked people's data.

Why get a warrant when you can just buy it?

This whole data economy has significantly undermined privacy, including 4th amendment protections.

Admissible evidence probably requires parallel construction and then a warrant. The purchased data is the catalyst but not legally actionable.

Parallel construction like that is unambiguously fruit from the poison tree. It should never be allowed, and the fact that it is used routinely is one of the many ongoing travesties in the US.

My understanding is that it would be, if admitted to. That's where the parallel comes in: establish an evidentiary trail that's plausible enough to withstand defense scrutiny, and count on the court itself (ie, judge) not to dig any deeper.

Right, but since that's the world we have today, our threat models should all account for it until we can meaningfully change things.

That's the "parallel" part. They're using information that they aren't allowed to use but are constructing an alternate path to get to the same conclusion with information they could be allowed to use, even though they didn't.

Do you have a ruling that says they can't used purchased data?

How would they select which women get warrants served if they don't have some basic cycle-tracking data?

You can't (in theory) get a warrant "just because", you need to state the grounds for the search in your application for a warrant.

What reason would they have to ask for a warrant without that data?