See the big ol' ASC mart link at the top of the site?
you can buy BB bastard .25s for cheap from there. They are Canadian, and widely regarded, on our side of canada at least, as the best BBs you can get.
I would recommend something heavier then .20s for outdoors though.
Theres also a few guys who sell BB's locally, but i don't think either of them are coming to the 21st.
It's better to light a candle, than to curse the darkness