Try www.goodfellow.com

Here is the NIST website for metals.  I notice that Ta isn’t certified, but you can bet the numbers are vert, very close to accurate.   There is another source I think is the Swedish Institute of Metals.

https://srmors.nist.gov/BrowseMaterials.cfm?subkey=1&category=1

It has been my experience that your are lucky just to get repeatable analyses of 17/4.  Ta is bear to analyze in a stainless matrix.   We had a lot of components “Tantalated” and tried very hard do analytical work on the materials with very little success.

Unclesyd's comment on the difficulty of tantalum detection in a stainless matrix is one of the reasons that Committee F of the SAE Aerospace Metals Division has removed tantalum from the composition list for 17-4, 15-5, and similar metals in the AMS series of specifications.  That, and the fact that since Ta is so much more expensive than Cb, the actual amount of Ta found in Cb ferroalloys is slight.

So check out AMS 5643 for 17-4 bar, for instance, and you'll see that Ta is no longer required to be analyzed.  If you buy to ASTM specifications, perhaps an e-mail to the proper committee would cause them to review the situation.

Good luck!

Lee