Frazer Brothers sutler is a very nice shop, and worth a look. The site as a decidedly western feel to it, and includes items from many time periods, the Civil War being but one. If you're into the Spanish-American War, this is one of the few sutlers that cater to this time period.
The Web site is great -- very graphical and easy to navigate. The only drawback is that they're still working on some aspects of it, so not all the links are working. But, that won't stop you from accessing pages, you may need to click an extra time to get where you want.
As with any sutler I haven't personally bought from in the past, I can't personally vouch for the quality of the items that Frazer Brothers sells. So, as with any sutler, I would collect some recommendations before ordering. Many groups sites do recommend Frazer Brothers as a primary sutler.