Cheap BMX cog?
 

Is there any such thing as a reasonably priced BMX cog larger than 18 teeth? I use a 42 tooth chainring on my singlespeed MTB with an 18 tooth Shimano cog but that gear will be too high once it starts to snow. Surly and Chris King are just too expensive. Thanks.

ACS makes them. there are others out there, but be careful, the cheap ones don't always have provisions for a freewheel removal tool.

I'm looking for just the cog or sprocket which I put on a normal cassette hub.

Unless you insist on 1/8", just take apart an old cassette and use the cog you want. Cassettes are usually held together with rivets or tiny allen bolts that can easily be removed, plus you can save the spacers and other cogs for future use.