You can use the 7D built in flash to trigger other (canon) flashes but you're better off with triggers IMO - more reliable. RF602s are ones a lot of people use and are very cheap. If you're doing everything manual you can get the cheapest Yongnuo flash, you just need to be able to adjust power. Being able to tilt/swivel/zoom the flash is preferable though.
If not using triggers you can use the YN flashes in a dumb optical slave mode but I think there might be some that work with the 7Ds built in flash to let you adjust power etc remotely - not sure