If I wanted to plug my MP3 into a stereo with an Aux port, what kind of cable could I use? The MP3 takes a standard headphone jack. If you what kind of cable I need where can I buy one? Thanks.

That depends on what type connectors the AUX port on the stereo accepts. It will likely be RCA or mini-stereo.

What size is the standard MP3 jack (mm)? And how do you find out what kind of jack the stereo accepts?

Usually 3.5mm and you look on the back or in the manual.

What do you recomend I do if the stereo I have is an older stereo (The manual is probably very deep in the land fill)? The stereo doesn't even have an Aux port.

Maybe this will fit your needs

Amazon.com: 1/8" Stereo (3.5mm) Male Mini to 2-RCA Male Audio Y-Cable iPod Compatible (6 FT.): Electronics

The stereo doesn't even have an Aux port.

Ummm, in your opening post, you said,

If I wanted to plug my MP3 into a stereo with an Aux port, what kind of cable could I use?

Mixed messages do not make it easy on us.

RCA connectors have been around for decades so my "guess" is the cable roky suggested is what you need. But, your stereo must have some "line-level" input port for this to work. That would include AUX, Line In, Tape In, CD In, or something similar. Microphone In and Phono In are NOT line-level inputs, so don't use them.

Originally I ment my car stereo which has an aux port. However, I also would like to attach it to an older stereo I have. Sorry I didn't make that clear. The cable roxy suggested looks like it will work, thanks to the both of you.

No problem. But I note a car stereo would typically use the 3.5mm jacks where home stereos typically use RCA, or more recently, some form of digital connection.