Velosolo is a good source of fixed/single speed stuff.
If the bike comes with a 16t, worth stocking up with a 17, 18 sprockets. With horizontal fork ends, you can usually cope with a sprocket difference of about 4 teeth, without adding or removing chain links.
Also a chainwhip if you haven't got one already and a nice spanner for your saddle bag to cater with the wheel nuts. Should be 15mm, but older imperial wheels were 1/2" and 9/16" sizes. A small adjustable spanner is a useful addition to carry with you on the bike.