Sounds like nothing but adjustment needed. You shouldn't need to be constantly adjusting the drivetrain to get it working correctly. You don't state what components you have now, but parktool.com is a good source for advice on repair and maintenance is you are doing it yourself. If the bike is new, you will have some cable stretch at first (they all do), and adjustment is just part of the routine. Don't know of anyone making mtb friction shifters-but once the adjustments are set up correctly, you wouldn't need them.