If you look at a table like this ...
... you can see that any of the chainring tooth counts in the center will be the most flexible in terms of attainable gear ratios. For example, if you get a 46t chainring, you can get a 14t cog for 88 GI which would be considered relatively high, or you could get a 17t cog for 73 GI which would be considered relatively low. These cogs in these sizes are likely widely available at your LBS ... and in several other sizes no doubt. My LBS (that does not specialize in fixed gears) sells cogs anywhere from 13t to 22t as far as I know, so you should have no trouble finding a cog for a 46t chainring. Any chainring with roughly the same number of teeth will be equally flexible in terms of gear ratio. Some people like a 47t chainring because it is a prime number, making it ideal for skidding.