Can I purchase FrameMaker 11 through Adobe?

CDW still offers it:
Adobe FrameMaker ( v. 11 ) - license - 65186931AA03A00 - Desktop Publishing Software - CDW.com

It turns out you can go the link below to download the trial & then it will give you the option to purchase. I am in the process of attempting this.

Download FrameMaker | 12, 11, 10, 9

Possibly - if you are already part of the Volume Licensing arrangement, you can add more seats to your existing license. If not, try contacting FM support - they may be able to get you a FM11 license.