Southwest Strings - another source for sheet music & accessories that periodically offers discounts.
Operated by Hawaii Music Supply, this website has an incredibly wide selection of professional ukuleles for sale. In addition, they boast something not all online instrument retailers can: they have a Youtube channel through which you can hear each instrument and get an idea of its timbre and quality. (This link remains as a great resource, even though I am no longer offering ukulele lessons for the time being.)
My personal go-to source for sheet music, books, and instrumental accessories. Sign up for their mailing list to get access to helpful discounts!
Additional Resources
Student login
An excellent resource for beginning ukuleles and string accessories.