You cannot buy miles from CO unless you are within 30% of your desired milage redemption level. You can ask DL to book flights on CO, but that is open to CO's policy on ST partner redemption levels.

Good luck, but I hope that CO is as stingy as DL on int'l reward flights -- which is slim to none......

You might have luck with a search on "DL SkyMiles for CO flights" or something like that.