A quick search revealed that this particular microwave does have parts in stock at a number of vendors! I second oldturkey03's advice to leave it to the professional, microwave ovens store very high voltages in large capacitors, and have killed before.
If you feel up to the task however, you are looking for Part Number PS237772 - or item number 43 on the following diagram:
[http://www.partselect.com/ModelFrames.aspx?ModelID=414447&ModelNum=JVM1440WH04&mfgModelNum=&ManufactureID=2&Selected=00119892i01&Position=1&mfg=GE&Type=&Mark=1|PartsSelect.com Diagram]
Keep in mind that some troubleshooting is in order, the drive motor is usually activated by a relay on the logic board, so be sure to check that as well.