I went to my local mtb shop and picked up a few used cassettes for free (he was going to trash them).
I got the spacers from them by grinding the pin heads off.
For the cog, if you are going all single speed (the larger chain) you could get a surly god, those things looks FAT. I got a cheapy 5$ ss cog, works well so far.
On a friends bike we used the original derailer as a tensioner, and aligned it with a short piece of cable, and a nut+bolt.