Found bmx chainring spider that may be an alternative. You can use 110 bcd chainrings with this adapter, so your options would be 38 to 48 tooth sprockets.
I found it on Ebay by searching for "chainring spider".
Both Harris and Pork Chop recommend BMX chainrings for single speed in their product descriptions of the chainrings, as opposed to a road bike 110 bcd chainring.
See the Orgin 8 BMX chainrings (110 or 130 cranks) for $17.96:
Here's the adapter on found on Ebay:
The product description states it work with 23.75mm spindles for one piece cranks.
Pork Chop also sells the chainrings in different colors, but are more expensive. I am sure you could find others.