Hey there. This is the forum for actually commissioning new maps from folks. I that is indeed what you're looking for, you should read the sticky post for details on how to do that. Otherwise, you might consider searching through the finished maps section for already completed stuff that will fit. Western doesn't seem to be a massively popular setting, but I know settings like Deadlands are pseudo-Western, so you might have some luck.

Also, if you're interested (to do a little blatant self-promotion), I've recently started a line of "Western Maps" on RPGNow for relatively cheap, so you can look there. It might be cheaper than commissioning new ones. You can also look under the map section filtered by genera on there, but there isn't too much western themed stuff up yet (It is a gap I'm trying to fill!)