Hi Eddie,
You can find chain and grass pickerel for sale but they'll be expensive. It would usually be from aquarium enthusiasts who have collected them from the wild.
I'm going to be stocking chain and grass pickerel in my girlfriend's pond at some point. I'll probably get a sturdy net and wade in some local creeks and rivers. At certain times of the year you can just run a good net into the weeds and get fingerlings.
