We live in a very small city and take our car pretty much anywhere, SO we have a fully stocked diaper bag that just stays in the car all the time. We are almost always close enough to the car to change a diaper or outfit or whatever. Then we can take a much smaller bag with the bare necessities if we're going in somewhere. I cannot believe that we didn't just do this with our first child (we lugged the big ole diaper bag around everywhere)!
I'm not sure if you have this luxury, since you live in a much larger city (as in I'm not sure if you take other modes of transportation to get places).
If you won't have your car nearby, then I would just buy a regular backpack. It doesn't have to say 'baby' on it for it to work for your needs. :) Hope you find something that works well for you!