They are made for ENGL by Celestion. You can buy them from ENGL, Germany (they should have them in stock).
Also, shipping could be expensive, depending on where you are.
I have an Engl Standard and a Engl XXL cab. The standard does indeed have the v12 60's in it. It doesn't say Engl on mine anywhere. Celestion stopped making the v12 60's a while back and started making them again for Engl. It's not a "engl design" speaker. It's the Celestion v12 60 but it's only made for one company. Engl is the only place you can get them unless you find a private seller. Engl is gonna charge you out the ass for it though and shipping is going to kill you if you live anywhere outside of Europe.