what kind are you mostly finding? have you tried going to a park or source of water that tends to give a different variety also having incense active gives you a chance of getting random pokemon not in your area
Pidgeys, Ratata, Drowzees, Jinx, etc. Been in 4 different towns and also in special places like the zoo and parks. With incense so far I have had one new Pokemon, a Voltorb. Lures have yet to give a new Pokemon.
eggs are probably the best way to get a variety of pokemons.
I think if your city lacks some kind of Pokemons than it's just very hard to get those. You just have to keep playing and hope they show up one day.
56 at lvl 19 is not even that bad though.
And always remember that this game has just been out for 2-3 weeks. No need to get all 150 that fast. Just keep enjoying the game.
Eggs get me Pidgeys, Zubats, Eevee, Magikarp, etc. Had 4 special Pokemon pop up so far in eggs and hatched at least 30 if not more.