C02 for outside pond

I would like to have a supply of aquarium plants growing in an outside pond
(not in direct sunlight) so that I can have large bunches of a few species
in the tank at any one time.  (At the moment my tank looks a mess because I
must have 20 or 30 different types of plants that have been given to me and
which I do not want to get rid of - and I cant afford any more indoor
tanks).  So, my question is, how would I put CO2 into a large pond (160 -
200 gallons) economically?

Thanks in advance