Which ones? the first set were purchased on Ebay for $5.00. I see them all the time going for about $15-$20. Just look under the equestrian category. Most of them are in the Buy-it-now store area.
Spend a bit more and you can get some pretty nice ones.

http://search.ebay.com/search/search...=32%26fsoo%3D2

As for the black ones, they are Willie and Max motorcycle panniers. Got mine locally for $60.00(used) I think.