I got it at LBS, it was only a couple of quid for the clamp and you can fit a standard bottle cage to it. I also made one for holding a bottle under the saddle out of a piece of sheet steel bent into a trough shape, with the sides of the trough sitting through the seatpost bolt, and the bottom of the trough having two holes in it through which a standard bottle cage was bolted. Unfortunately I lost it so I'll have to make another one - it only took about 15 minutes though.
I got it at LBS, it was only a couple of quid for the clamp and you can fit a standard bottle cage to it. I also made one for holding a bottle under the saddle out of a piece of sheet steel bent into a trough shape, with the sides of the trough sitting through the seatpost bolt, and the bottom of the trough having two holes in it through which a standard bottle cage was bolted. Unfortunately I lost it so I'll have to make another one - it only took about 15 minutes though.