David   G0MRF said:

I recall seeing several mails in the past talking about large verticals

lifted by balloons.

The problem was always where to buy the gas.

I did a lot of experiments with balloons and kites some years ago and found
balloon gas easy to obtain locally. Every florist shop has bottled gas to
inflate party balloons that for some reason seem to be sold from the same
place as flowers.
Yellow Pages is a good start for finding a local supplier

Peter, G3LDO