Because you asked...
Here are a few links from those that have made their own laundry soap:
Click here to read how the Domestic Diva Lindsey gets crafty.
And here to read how New Yorker Jacqualine does!
I buy my detergent from Nurtured Family and use Allen's Naturally Detergent - 5lb Powder {though there are others to choose from}. It is generally just under $20 a box and lasts a looooong time, approximately 8 months or so {$2.50/mo}!