As mentioned, it depends.
If you want the 'classic'
GW Dwarfs, ebay is your best bet but expect to pay a premium at the moment. If you aren't that bothered about that, you can either convert out of
GW models or search on Google for blood bowl dwarfs and choose a third party manufacturer. Personally if you wait a few weeks, we may have the new game released with the new Dwarf team!!!!
Image from War of Sigmar site - Picture taken at SPIEL event over the weekend