I have a cooper.... Can I install a Cooper S exhaust on a Cooper but of course I'm gonna change the bumper. So is this possible and where can i buy this exhaust?

 

Anything is possible. Just be aware that some modification and fabrication will be in your future.
First, you'll have to deal with the spare tire. Easy enough to get rid of, but will you be comfortable without it?
Second, since the S exhaust has 2 cans, you'll need to figure out a way to attach the new set of hangers.
Are you looking to put an aftermarket exhaust on the MINI, or a stock MCS part?

 

Just get the Megan Racing Cooper exhaust, you'll like it. fitting an S exhaust on a Cooper would be a royal pain.