Don't order any online or bring in exotic mussles into your pond. you will open a pandoras box of overpopulation or no success at all. Use some from the local lakes but be warned they will most likely do very well, maybe too well. -phillip

p.s. not that it would likely do much harm but north american mussles and many exotics are parasitic to fish in the larval stage.
google: glochidia