Try Dittmers in Mountain View too. They list garlic sausage on their website, although I haven't had it myself. I've tried several other sausages they make and can vouch for the overall quality of their stuff.
I haven't tried the garlic sausage there either, but do remember that Dittmers is German (though they have a wide range of sausages that goes beyond that). So don't get your hopes up too high in case it's not quite authentic.