Trade Help

I guys, doing my first play through of Heartgold. I know the Nintendo DS online is no longer functional for these systems but is there still a way to manage a trade? I'm just trying to complete the story but I would love to evolve my haunter into Gengar before taking on Elite 4 the second time. Any help would be greatly appreciated. I love haunter but I always had a Gengar on my teams growing up and I'd like to have him again.