Highly unlikely you will find FM9 for sale through Adobe - some resellers may have old copies/serial numbers out there. Why bother with such an old version when you can get FM2017?
This is not my requirement but some are clients want only FM 9, already I have FM 2017, because of I am going to buy this one.
If they already have existing FM9 licenses, and they are covered under the volume licensing plan, they may be able to contact Adobe to obtain more FM9 licenses.
Mirroring Jeff's comment, finding a retail copy of Fm 9 may prove difficult.
However, if you find a private sale of Fm 9 (eBay, etc) you can use the information at Transfer an Adobe product license to officially transfer the license to you.
Asking the seller to fill out this form is probably the easiest way to ensure that your software purchase is legit.
The usual go-to source for legacy FM licenses and media is CDW dot com.
They presently only have Windows versions back to FM 11.0.
(The FM8.0 they have is for Solaris Unix, still around because it was the final version for that platform).