http://WWW.partwizard.com/

http://www.appliancefactoryparts.com/

http://www.appliancepartspros.com/

http://www3.sears.com/  Never buy a part from Sears online unless you can't get it anywhere else.  I ordered a refrigerator part from them.  When it hadn't arrived in a week, I called.  I was handed from person to person, holding in queues for each person.  Thirty minutes of waiting later I was told the part had been shipped.  When I insisted they tell me when it was shipped they changed their story and admitted it had not been shipped.  When I tried to cancel the order they refused.  Based on what they told me, it normally takes them 5 days to ship.  I ordered from another place who shipped the same day.  The Sears item arrived 3 weeks after the order.

This one has parts lookup and repair manuals online.

http://www.appliancepartsworldwide.com/

This one has parts lookup and an appliance forum

http://forums.partselect.com/

The following places claim to help you with troubleshooting and part number identification.

http://www.pcappliancerepair.com/   

http://repairclinic.com/  I asked this place for help finding a part number for my failed air conditioner indoor fan relay.  When they didn't reply in two days (other than an autoreply form letter saying they'd answer within 24 hours), I sent a follow-up with my case number asking if they intended to help me.  They never bothered to answer either inquiry.  I won't be using them again.

http://www.easyapplianceparts.com/  This place was cheaper than the one immediately above for a Jenn-Air range burner switch.

The following place sells parts for gas grills

http://www.clagrills.com/

There's an off chance you can find your part by putting the part number in a few internet search engines (like google.com)