Earth Science makes an olive oil and avacado shampoo and conditioner that is great. It's $7 for about a 6 oz bottle, but it's very concentrated, so you don't have to use a lot and it lasts a while.
If you have dry hair, like I do, I'd also recommend using oils (jojoba, coconut, etc., depending upon what works for you) as a treatment before shampooing.
It's also pretty easy to make your own dry shampoo using corn starch or tapioca starch (or cocoa powder, if you have dark hair) and a little essential oil for fragrance. There are different recipes and instructions out there for how to do this, if you google it.