seems they come out of different parts of north america and different regions get different sizes. I would say a great option for a cold water tank/pond. Not to mention they WILL eat you out of house and home and grow faster then you can shake a stick.
In the yellow pages they list fish farms that sell to people stocking ponds and all that. I bet you could get them extremely cheaply from a fish farm, especially this time of year as it is getting colder.