If your local plumbing supply store doesn't have the required length, they may have a nipple threading machine, and can make one for you. If not, you should be able to find one on-line. WOrse comes to worse, you cut it off and buy a die and thread one yourself. This isn't a code issue, so buy or make what you need. You might try two short nipples with a coupler to see how long it is. The two extra sets of threads give you a little more leaway in exact length.