The ones we were using came from Dr. Foster and Smith. You might be able to get it at one of the big stores that also does pond stuff (aquarium adventure, living sea). Here is the link to the wedsite. They have two types- one that is water tight and one that is not. We used the one that is not (thinking that the net could hold 10 gal of water- a lot of weight when trying to move fish quickly). I think both would work well.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=12291&Ntt=sock%20net&Ntk=All&Ntx=mode+matchallpartial&Np=1&pc=1&N=0&Nty=1
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=12291&Ntt=sock%20net&Ntk=All&Ntx=mode+matchallpartial&Np=1&pc=1&N=0&Nty=1