If there's any turning-related tools or supplies sites you've already found (and hopefully bookmarked), you'd be surprised at the number of 'em that also sell wood for all types of projects--including a wide range in dimensions suitable for tool handles. The hard part is deciding between "basic durability" wood types for hard service use or economic reasons, or whether to step up to something a little nicer that'll be noticed by your fellow turners the next time you get that tool out to use it.
IIRC, Packard Woodworks and Craft Supplies both include ferrules when you buy tool handle blanks, but there's no shortage of other places, either. A good search engine can easily be your friend on this one.