I've heard agents say at conferences that you should definitely have a finished manuscript before querying. You shouldn't send the whole manuscript, though. Check out exactly what each agent requirements. Usually their websites will tell you. Some may only want a letter; some may want 50 pages. Regardless, the are FLOODED with queries every day, and you don't want to be dismissed because you couldn't follow their guidelines.
Agents who do urban fantasy off the top of my head: Donald Maass Agency (Jim's), Ashley Grayson Agency, Rachel Vater at Lowenstein and Young (think I'm spelling that right--searching for Rachel Vater should pull her), Spectrum Literary Agency, Ethan Ellenberg Agency, Folio Lit Agency, Jabberwocky Agency, and...that's all that's coming. You can find all of them on Google, but again: read their guidelines and make sure you query the specific agent who handles this genre.