Beware of the lowball vendors... they'll do things like upsell you on the battery, charger, etc. Go with reputable companies. Here's who I use, in no particular order:
www.durys.com (currently my favorite store in Nashville)
www.bhphotovideo.com (the king of mail order in NYC)
www.calumetphoto.com (keep an eye on their used gear.. main store is Chicago)
www.tallyns.com (they sell just about everything except Nikon)
www.robertsimaging.com (never used but understand they're very reputable)
Now you'll laugh about the best place to go look for quality used gear...
www.digitalweddingforum.com .. thousands of wedding photographers who have their reputations to uphold when selling used gear.