Just gotta start looking around. When I first started it took me a while to find certain providers that had what I wanted at a price that made sense. Until then, I was either buying online or using other products that were not as good, no other way around it. At first I was using Red Devil Hydroxide for $100 per 50lb bag. Eventually I found a local guy that I can get it for $50 for 50lb.
The biggest challenge imo with online ordering is the shipping cost for chemicals. It seems to make more sense when you can buy in larger volumes (even 5 gal container verses 1 gal is a better deal after shipping per gal, but a larger upfront out of pocket cash). This is also true with in-store purchases. When possible buy in largest quantity you can that makes sense to lower your per unit cost (lb, oz, etc) on things you use a lot of.
Look around for power washing vendors. Sometimes I will find good stuff at Janitorial supply stores. You can check with Sherwin Williams - I think I mentioned this before that they used to carry it at all stores then stopped. I found one last week that still has 5 gal containers for $125. That is $25/gal, much better than the $43/gal from online.