I have an old Stanadyne (I've been told this is now Moen) bathtub faucet that leaks out of the spout when it is turned off. I'm not sure how to describe the faucet other than it is a single-handle that I pull away from the wall and then turn right or left to adjust the water temperature. It looks as if there is a cartridge inside. I wonder where I can get parts or should I just call a plumber. Thanks.

Moen has great tutorials for this and the parts are free under their life time warranty. Read the tutorial and then look for the site to order warranty parts. I am doing this same job tomorrow but can't wait for the freebie. Installation Help / Animated Tutorials for Moen Faucet (http://www.moen.com/consumer-support/installation-help/tutorial?id=moen0133)

Bob gave you an excellent link, click on the link go all the way to the end of the page and click on the Watch Animation when it opens click on the start, go next and find your faucet and from there it will show you step by step how to replace the cartridge, it looks like your faucet is 4a, as Bob says call Moen at 800-289-6636 will send you a new cartridge with no charge to you, good luck.

John