@ lillimere I'm using Jason Health for both ctx and p1np. The thread suggested by @mayblin has lots of info. It's the cheapest place I've found. The ctx is $50 plus $18 fee for prescription-lab draw etc. The $18 is a once a visit fee. If you had 10 tests in a visit it would still be $18. The actual blood draw takes place at a Quest site in my area. Oh, P1NP is tricky to find on Jason Health. I had to call and get help from their support folks.

My doctor told me that traditional medicare will only cover ctx once a year. If anyone knows anything different than that, like how we can get Medicare to cover it more often, please let me know. Otherwise it's worth it to me to pay myself when I want the tests done.
My situation is a good example of when you might want to do frequent bone marker testing. I'm testing monthly right now to see if Risedronate can hold my Evenity gains. I don't want to wait months to find out it's not working. I want to find out right away and change meds if so. There are studies with Risedronate that show how ctx responds to it at selected time periods. It should go down right away, like in the first couple of weeks. That's going from no meds to Risedronate not from Evenity to Risedronate but still there is data showing the ctx should go down fast. Testing ctx monthly in this case is the kind of info that is invaluable but is not what most of us are getting.