Silvergen and Silver Puppy seem to be the makers of choice but they're both quite pricey.
Personally I just put two silver rods in a large preferably glass container (at the moment I'm using plastic but I know that's a bit naughty lol), connected to 4 9v batteries, and make it up to about 13-15PPM then let it settle down to about 10-12 once the larger particles have dropped to the bottom.
I check the PPM with a TDS meter I got cheap off ebay :)
I find of course the further apart I keep the electrodes the slower it is but the better a solution I get, particularly if I keep it moving. I might grab a fountain pump sometime to aid in this as again they're nice and cheap on ebay! ^^