Udon sans Dashi
I wanted to make some Udon noodles this weekend but realized that I haven't got the stuff I need to make dashi. I'm also out of miso.
Any ideas on substitutes for the broth that would still taste similar?
You included information about what you don't have, but nothing about what you do have. A blend of Kombu or similar dried sea weed product and some flaked dried fish will provide you with a Dashi base, or you could try some fish stock or chicken stock and sesame oil. But neither is going to have the wonderful flavor profile of Dashi.
You could make stock with niboshi (baby sardines). Or go a different direction and make kamagge udon (hot udon with dipping sauces) or the cold version zaru udon, curry udon or yaki udon.