Micro-Mark is another source. Here's a page with a search of their catalog, on keywords, "brass tube":
http://www.micromark.com/SearchResult.aspx?deptIdFilter=0&searchPhrase=brass+tube
You can buy specific sizes, or even get an assortment pack.
If you still haven't given up on buying from a store near you, though, I recommend looking up model railroad shops. If there's one near you, it's likely to carry brass tube and brass rod stock.
Hope that all helps!
Best regards,
Brad