Try not to post more than once for the same question - i'm sure it was a mistake.
They dont' sell a lot of great products on this site, i'm sure its a supply and demand thing. They don't sell (used to though) refractometers either. And it is a great plant for a fuge, but there are a lot of other great things to add, ls, LR rubble, and other plants if you want. Really chaeto just soaks up some of the nitrates - it by not means in a cure-all and as "magical" as you may think it is. Just a good mixture of LR, chaeto (or any other nitrate feeding plant) will reduce your nitrates.
My LFS carries a ton of it, he doesn't really sell it per say - just ask him for a chunk and he'll hand it to you. I've heard this stuff grows extremely fast and you have to constantly trim it, so maybe someone online will sell/give you some if you pay for shipping.
And i can't show you were to purchase chaeto b/c the rules of this forum, just google it.