I buy mine at the thechemistrystore.com
http://www.chemistrystore.com/cart.cgi?group=100513&child=100614
I use it in my laundry as well because I have hard water.
If you want to buy “oxyclean” without fillers and perfumes buy sodium precarbonate.
http://www.chemistrystore.com/cart.cgi?group=100513&child=100581
There are also a few DIY soap stores online but never tried them. The Chemistry Store has been very reliable for me and very price competative when shipping is included.
Sodium tripolyphosphate (STPP) from TheChemistryStore works great.
I use it both for dishes and clothes.