Tamiya has some nice washes called Panel Line Accent. They are available in black, grey, and brown. They are enamel, so not recommended to use over other enamels.
I frequently make a wash by mixing acrylic black with alcohol, about 10:1 alcohol to paint. You cane make it more thin or less thin to suit your purpose. Gloss black makes a smoother flowing wash , but flat black often makes for a better appearance when dry, depending on application.