I did the same thing on my phase i went online to superstar components they have the sprockets i use a 20t but if i were you go for the 18t
its £10 for the sprockets and spacers, there brill too, i also bought the tensioner but a word of advise dont buy it, it doesnt work + you dont need it just take links out for your chain, its a bit tight but i works fine.
I also bought the cassette tool for £5 i fitted the single speed kit my self, i saved a lot of money and i have had no problems with it at all, it was a great buy.