There is a decentralized MSC exchange in Alpha stage at the moment, which will go live soon.
If there is a MSC exchange currently I have to apologize. However, afaik there are only being test msc traded currently. Is there already a release date when there will be real MSC being traded?
I believe there was an ETA of 2 months given back in November, so launch date is drawing near.
In the meantime, people can use the
https://masterxchange.com centralized exchange to buy and sell Mastercoins (MSC). Works quite well. They have also increased the bounty to $10k for the first existing exchange that accepts Mastercoin.