I got my bike second hand, and among the things I fitted to it, were lights, a rear mudguard, and a Bell! I have no idea how many bikes are sold without bells nowadays, but I am convinced that all bicycles should have some audible warning device. A bell is perfect, vert easy to operate, small, cheap to buy, can be heard from quite a distance, and will not be mistaken for anything else. Also, the sound is very "friendly", it says "Excuse me" rather than "Get out off the way".
These bells are available all over the place, for as little as 99 pence. They are made in the shape of Soccer & Pool balls, and even Ladybirds! I don't care what they look like, I have one because they are very practical. I fitted an old second hand one from a previous bike.
You will find many bells on sites like ebay, or in department stores, and corner shops, and of course cycle shops. I suggest that you have a think about your bike, have you got one? I can recommend them.