I get ours a variety of places depending on what is on sale. I stockpile and my goal price for laundry detergent is about $2 a bottle for a 32ish load bottle. I am not brand specific but don't buy super cheap brands. I can usually get ALL and Purex at this price point. Most often I buy it at Dillons (our Kroger affiliate) and lately CVS has been doing a buy 1 get 2 free sale on Purex every few months.
Ha! We should live next door to each other. I have bought regular detergent for my HE washer not once, not twice, but THREE times. I am an idiot. I discover the mistake when the washer overflows and I have a huge soapy mess to clean up. I could have given you all my barely used bottles of detergent!