DMR sells a singlespeed kit that comes with spacers for like $29.95 or something I think. Spacers and a really ghetto cog.
I ran mine ghetto for a really long time, once with my cassette and I just taped up the gear bigger than the one I was in so my chain didn't jump up and get super tight. Then I took the cassette apart and rearranged it so there were 2 big cogs beside my chain kinda like a guide. Now I run it with just the one ring and spacers.
If you have 2 old cassettes, pry them apart and take the spacers from inbetween the cogs, it'll work even better than the little kit from DMR.
New stuff isn't really needed. And singlespeed is awesome, you can seriously feel how much more efficient it is, it's weird.