I buy my haircare products at costco and ulta.
At costco some products are steady others are seasonal. They carry Nexxus regularly and nioxin and Biosilk sporadically. You get the huge sizes for half off the salon price. When I see these I stock up.
At ulta I look for sales around holidays,mothers day and at the end of a season on fancy hair products. I wait till their 50% or come in a value size or in a value set. this saves me money,plus the sets come in a tote bag that I can reuse for books groceries or as a gift bag. And I earn ulta points towards a free or reduced haircut or eyebrow wax or some other cosmetic item.
Holidays and mothers day are the best times to find deals on haircare sets of salon brand items. These items are good for a year or more(sometimes two) so if you stock up when its cheaper and its an item you regurarely use you save money throught the year by not having to buy the item later at full price. I also love buying the holiday sets that include free mini sizes of the product in a free gift bag with the purchase of the larger sizes. I use the free mini items and gift bags along with bubble bath and body wash I found on clearence during the year as a great gift to give away at christmas time.