Unfortunately, a lot of what is toted to be 'real' in terms of designer purses on ebay, is in fact, knock offs. If it seems too good to be true, it is. LOL Esp when it comes to ebay. I would never buy a designer off of ebay, I just don't trust people all that much. LOL
I've seen the knock offs in large cities like Boston, Chicago, NYC, Miami, LA, San Francisco etc in the downtown areas, usually at those little cart kiosks, and then sometimes at malls. It just depends on your area. They also can be pricey, 10 years ago you could get a knock off purse in NYC for $5, now it depends on the style of course, but they're a lot more expensive than they used to be...more people want them too, so I guess that makes sense, supply/demand.
You can also find them online, I'd just try googling words like "replica" "designer inspired" "knock off" along with the designer name you're interested in.
I did a quick google and found these:http://www.replicasi.com/default.asphttp://www.fashionheaven.com/http://www.fashionknockoffs.com/