Marc by Marc Jacobs

MBMJ is not a cheap brand so you are not going to find authentic bags from them 'cheap' anywhere. THat being said you should check ebay because they do have many NWT bags. Some other places to look are saks, bloomies, and piperlime. One way to get a bag like this on a discount is to buy it in a Friends and Family Event at Saks or Bloomies. You can get it for 25% off. These sales are in the spring and fall.

ETA: MBMJ is not a huge counterfeiting brand so don't stress too much about an ebay purchase.

Zappos.com is the best shopping site out there. It's all real and they have MJ and have lots on sale too.

If you find one on ebay make sure it's the real deal. It would be sucky and pointless to give some a bootleg designer purse.

sign up for emails from Nordstroms. They always have sales and you can probably find one at a discount on their website.

Ebay. Or Hautelook.com has a lot of sales like that, but you have to wait and watch for it. They have different sales everyday.