Use a tubing cutter to to cut the copper pipe and get the old flare nut off - cut as little as possible, slip on a nut and flare the cut end. That will likely leave you a tad short on pipe length. There is indeed a flare-Tee. One possibility is to use one at this point (plus a male to male joiner) to make up for the piece you cut off. The tee would, of course, need a valve or a cap on the open leg.
My source of flare components is our local supplier of air-conditioning parts.
Use the standard flaring tool available from just about everywhere - single flare for copper.
To get an idea of what flare fittings are available, take a look at Plumbing Supply