Discount Aquarium in Tempe has a whole room full of second hand equipment and usually includes a big selection of dosers. Some of the equipment is obviously beyond it's expected life and is good for only parts but a lot of it is good stuff. We try to stop by there anytime we're in the area as the stock changes weekly. You can find some second hand diamonds in the rough for dirt cheap sometimes. David the manager is very lenient with used stuff, if you get it home and it doesn't work, he'll usually make it right.
Waterlogged and I stopped by Discount 2 weeks ago and I found a used Spectrapure LiterMeter III on the shelf with no price on it, I asked David if he'd take $25 for it as it had no power supply and I couldn't test it, he chuckled and said that's way too much he'd take $20. I got it home, plugged it into a power supply I had laying around and it works perfectly. They don't make LM III any longer but Spectrapure will still service them, they were $400 when new.
Moral of the story, Discount may have something you could use.
