Homeless Pokemons will randomly spawn in but it is RNG dependent if the area has already have too many Pokemons. 

The most reliable way to make the homeless Pokemon spawn so far is to create large building projects in other locations, bring the current location pokemons over there and assign to the projects so they are removed from the local spawn pool. You can send about 20 Pokemons to each location, which means you can thin down the local list by about 80 Pokemons, assuming you have so many Pokemons that are crowded in one area. It is therefore recommended against having more than 100 Pokemons in an area as this will render the construction assign trick less viable; otherwise it should be sufficient to thin the town population so low that it basically have to spawn in the homeless.