If your freewheel cog or your chain ring is 1/8" thick, you need a 1/8" chain. These are sometimes sold as "track" or "fixed gear" chains. They are a little heavier than narrower chains but they probably wear longer. Some people tout their additional strength, but this is nonsense because there is no human alive who can break (through normal use) a properly maintained bicycle chain of any size.

If your cog and chain ring are 3/32" thick, you can use pretty much any modern bike chain. 5/6/7 speed chains work. So do 8/9/10 speed. (I'm not sure about 11 or 12 speed.) You can also use a single speed/track chain (1/8") with these narrow sprockets--there are zero problems with this. It really comes down to what you prefer and how much you want to spend. There are dozens of options available.