Coupons vs The Dollar Store

So Ive been wondering, whats so great about using coupons to buy things when I can get some of the things I want for a dollar at The Dollar Tree...? Ive been trying to use coupons for some things I get at the dollar tree but for less than a dollar and its just not possible in CA. 
Has anyone out there had any luck? Things I just havent found for less than a dollar that I need are Shampoo and Body wash. Where and I mean WHERE can I get these for less than a buck? 
As I said before my best deals Ive gotten so far from couponing are toilet paper deals. But when it comes to shampoo and body wash I just cant find that super cheap extreme coupon deal. 
Please leave comments if you have advice! Thanks~!