The electrochemical potential for the reduction of silver ion to silver is around 0.8V. So if you were to use a battery, or wall-wart style DC source, it would be best to arrange a series of cells in series to regulate the voltage in each. If you cannot get hold of silver wire, or sheet, to form the anode, you could use stainless steel (ie. any old unused stainless steel knife blades) in its place. If I remember correctly, silver plated on to stainless steel does not form a very strong bond, and can be removed by flexing the blade, or scraping off.
The gasses evolved using too high a voltage or amperage will contain dissociation products of the thiosulphate anion, which will be hydrogen sulphide - smelly, poisonous, and dangerous to unexposed photographic emulsions. Indeed this will probably take place in preference to the dissociation of water to form hydrogen and oxygen (though I would have to drag out the Rubber Handbook, and brush up on my electrochemistry to prove that one way or the other). :P