Whether you are just starting out and looking for a cheap starter bathing system or just want to have an easy and cheap backup in your unit, the Texas Bathing System is just the thing you need.  I cannot take credit for this as a groomer that worked from me told me about it and she learned it from someone she worked with in Texas, thus the Texas Bathing System.  There are several ways you can implement this, but I will tell you the one I use to use.   I would premix my shampoo to the recommended mix and fill the reservoir.  I suggest you might only fill it half full as it tends to be a bit heavy when it is full.  I set the dial to the lowest setting.  Once you start using it you can adjust the dial to put out the amount of shampoo you want.  There is an off setting on the dial, but I just spun the reservoir off to rinse.  That way I got the benefit of a pretty powerful jet of water for both bathing and rinsing.  Looks like they have an upgraded version as pictured below.

Note:  You can use the nozzle part as a great rinsing nozzle as it jets the water right down to the skin.

This appears to be the newest version of the one shown at the beginning of the post.